Light and Color - A Golden Guide

J1IIeillll� &NID CCJ (]) J1 (]) lli by CLARENCE RAINWATER Professor of Physics San Francisco State College Original Project Editor

HERBERT S. ZIM Illustrated by

RAYMOND PERLMAN Professor of Art, University of Illinois

�

GOLDEN PRESS

•

NEW YORK

Western Publishing Company, Inc. Racine, Wisconsin

FOREWORD

This Go lden Guide sin g l es out the phenomena of light and color and describes the scientific concepts in easily understood terms. Light and color are intimate ly invo lved in our lives yet a rea l understanding of their n ature is rare. This book presents in simpl e terms the complex physica l, physio logica l, and psychologica l aspects of light a n d color. To condense this subject into this sm a l l book req u ired some sacrifice, so many details a n d qua li­ fying remarks have been omitted, a n d m uch of the data has been presented in simp lifi ed form. We are grateful to the individua l s and organizations who generously sup plied data and loaned pictures for our i l l u strations a nd to the author s of the many excel lent books wh ich were drawn upon for idea s and i n fo rm ation ( B i bl iography o n page 1 5 6 ) . We are gra tefu l a l so to Ja mes Hath way, Ja mes Ske l ly, and George Fichter for th eir ed itoria l assistance a nd to Dr . Frederick L. Brown for his cri tica l review. Photo CNdits:

MI. Wilson & Palomar Observatories, Copyright b y California Institute of Carnegie Institute of Washington, 7, 74, 75; Clarence Rainwater, 30, 56, 79, 92, 93, 95, 97, 119, 144; Enid Kotchnig, 31, 146; Ealing Corp., 46; 0. C. Rudolph & Sons, Inc., 54; Yerkes Observatory, 72; lnst�ule for International Research, 96; Florida Development Commission, 99; Roger Behrens, 109; ''The Printing Industry" by Victor Strauss, 112; American Optical Co., 116; redrawn from Scientific American, 118; from "An Introduction to Color" by R. M. Evans, 120; Munsell Color Co., 127; Contai· ner Corp. of America, 1 29; painting by Louis M. Condax from "The Science of Color," Optical Society of America, 132; The United Piece Dye Works, 140; Western Electric, 142 (bot. left); Edward Diehl, 143; Original Dufaycolor by Blanche Glasgow, American Museum of Photography, 145; Elizabeth Wilcox (Polaroid), 147 Nati:>nal Gallery of Art, W ashington, D.C., Chester Dale Collection: detai l from Self Portrait, 1889, by Paul Gauquin. 149: Perkin-Elmer Corp., 152 (tap); Optics Technology, 152 (bot.). Technology and

;

GOLDEN,® A GOLDEN GutDE,® GoLDEN PREss® and GoLDENCRAFT"i are trademarks of Western Publishing Company, Inc.

©

Copyright 197 1 by Western Publishing Company, Inc. All rights reserved, including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge retrieval system or device, unless permission in writing is obtained from the copyright proprietor. Produced in the U.S.A. by Western Publishing Company, Inc. Published by Golden Press, New York , N.Y. Library of Congress Catalog Card Number: 69-11967 ISBN 0-307-63540-6

CONTENTS NATURE OF LIG.HT AND COLOR

.

.

4

.

.

18

Measurement, speed of light, electromag­ netic waves, spectra, rod iation LIGHT SOURCES

.

.

.

.

.

.

.

.

Sun, electric lights, glow tubes, mercury arc, fluorescent lamps, luminescence ILLUMINATION

Sensitivity

of

eyes,

brightness,

metric units, shadows, lightness

Ll G H T BEHAVI 0 R

Transmission,

.

.

.

reflection,

.

.

.

photo­

.

refraction,

.

.

29 36

dis­

persion, diffraction, interference, scatter­ ing, absorption, fluorescence, phospho­ rescence, polarization, double refraction

OPTICAL INSTRUMENTS .

Mirrors,

prisms,

.

.

diffraction

lenses,

aberrations,

scopes,

projectors,

.

.

.

58

gratings,

telescopes, enlargers,

micro­

cameras,

photometers, colorimeters

S E E I N G Ll G H T A ND C 0 L 0 R .

.

.

84

Eye, sight, depth perception, illusions

THE

NAT U R E 0 F C 0 L 0 R .

.

.

.

.

98

Hue, brightness, primary colors, comple­ mentary hues, additive and subtractive colors, color· matching, color blindness. COLOR PERCEPTION

Color constancy, contrast, afterimages

118 125

COLOR SYSTEMS

Munsell, Ostwald, CIE LIGHT AND COLOR AS TOOLS

Harmony and discord, symbolism, paints,

133

pigments, dyes, business and industrial uses, photography, printing, television, lasers, fiber optics MORE INFORMATION INDEX

.

.

.

.

.

.

.

156

157

Natural light dispersion-a double ra inbow

NATURE OF LIGHT AND COLOR

We know the wor l d th ro ugh ou r sen ses: s i g ht, h earing, to uc h , taste, and s m el l . Ea ch sense re spo n d s to pa r­ ti cu lar sti m u l i , a nd the se nsations we experience give us i n fo rmation a bo ut ou r surro u n d i n g s . Sight is the most importa nt of the sense s . Th ro ug h sight we pe rceive the shape, size, and co lor of ob jects; a l so the i r distance, motion s , and re lationsh ips to ea ch other. Light· i s the sti m u l u s for th e sense of sight- th e raw ma te ria l of vi sion. To u ndersta nd the fa scinati ng sto ry of l ig ht, l et u s ex plore its natu re , its ma nifestation o f colo r, its behavior in l e nses and prism s , and th en its uses i n science and art. This wi l l h e l p i n u nd erstand i ng how th e sen sat ion of see ing affects o u r· action , ou r attitudes, ou r moods, and ou r dai ly experiences. CO LOR i s t h e essence o f l ight; light the essence of li fe . The green pigment o f pla nts plays an essen­ tial role in susta i n i n g a ll li fe . The colo rs of many an i mals blend with their s u rroundings, h i di ng the animals from thei r enemies. Some, l ik e th i s anole, ca n even cha nge their c olors a s they move from one bockgrou nd to a n oth e r .

4

Man has put l ight a nd color to wo rk i n m any way s . Physicians detect d i seases b y changes i n t h e c o l o r o f eyeba l l s, throat, or ski n . The acid i ty o f a so l ution, the composition of an a l l oy, the tem perature of a fur­ nace, and the vel ocity of a distant star ca n be de­ termined by a color or a color cha nge . Decora tors choose restful colors for bed room s , brighter colors for work area s. In adve rti s i ng , a colo r entice s the consumer to c ha nge his b ra nd of b reakfa st food . Light a nd color give m ea ning to everyday contacts betwee n man and h i s wo rld i n many way s . L i g h t and color involve phys i ca l , physiolog i ca l , and psychological facto rs . Physici sts deal with the energies a nd frequ encies of light waves a nd the i nteraction of l ight with matter. Physiologi sts study visual processes and psychologi sts study the effects of visual and color perception. These th ree g roups of scienti sts developed different viewpoints and d i fferent voca bularies i n ta l king about light and color. After tong study, a comm ittee of the Optica l Society of America reconci l ed the d i fferences and set up a c lea rly defined and con s i sten t ter m i no logy.

A-,:{

[ t 0��� ]:1;.J

A prism disperses l ight to form a spectrum in a lobo- op" ·=--·- ' , ratory spectrograph j u s t os light dispersed by ra in, drops forms a rainbow.

··

H I LGER SPE CTR OGR APH

5

SCIENTIFIC MEASUREMENTS in­ volve large and small numbers, i nterrelated u n its, and great prec ision. Our everyday u n i ts of m easurement come from the E ng l i s h system with its i nche s, gallons, and pounds and are convenient to use only be· cause they are fam il iar. The met· ric system is favored by scientists beca use. the relation s h i p between units of length , volume, and

weight i s simpler. The syste m ' s u s e o f decimals a l s o makes for faster and more accurate computa­ tion s . larg e num bers can be e xpressed concisely. Metric units of length a re used i n thi s book . The table below l ists some common units, uses, sym bols, c om pa ra tive val­ ues in meters and in i nche s , a n d common objects of each u n i t ' s approx i mate size.

UNITS O F LE NGTH

Unit

Symbol

Equivalent in

About the

meters, inches

size of

MET E R mea sures rad io wav e s

m

1 m 3 9. 3 7 inches

A small boy

CENTI METER measures ' m ic rowaves

em

. 01 m (10-2 m ) 0 . 3 9 37 i n .

A s unflower seed

MilliMETE R mea sures m ic rowaves

mm

.00 1 m ( 1 o-3 m ) .03937 in.

A gra i n of sand

.000001 m ( 1 0 .0000 3 9 i n .

MI CRON mea sures i n frared MilliM I C RO N mea sures l ig h t waves AN GSTR OM mea sures u ltra v io let and l ig ht waves

6

m)

(.i

A small bacterium

. 00000000 1 m ( 1 0-9 m ) . 0000000 3 9 i n .

A

. 000000000 1 m ( 1 0-10 m ) A hydrogen . 000000003 9 i n . atom

A be"'e"" molecule

0

*

mp.

N u m bers in this book are often g iven as powers of 1 0 . For ex­ ample, 103 is 1 ,000 ( read 103 as ''1 0 to the third power' ' ;

6

-

(I

t h e 3 is cal led an e xponent), and 1 06 is 1 , 000 ,000 . Negative e xponents a re fractions or dec i­ mals; 1 0-3 is 1 11 ,000 or . 00 1.

The Andromeda nebula i s so far away tha t l i gh t from it takes about two and a ha lf m i l l ion y ea rs to reach the earth . Studies of l ight from such ce l estial bod i e s give cl u es to the structure of the u n iv erse .

OF LIGHT i n free space ( a vacuum) i s 1 86, 2 8 2 m i l es* p e r second . T h i s seem s to b e th e natural speed l i m i t i n the 1,mive rse . There i s g ood rea so n to bel i eve that noth i ng con ever travel foster. The speed of light in a vacuum i s a con sta nt, a lways denoted by c i n eq uations, as in E i nstei n ' s en ergy eq ua­ tion, E = m c2• No matter what th e source o f l ight, or how fa st the source and observer ore movi ng with respect to one a nother, the speed of l ight i n free spa ce is a lways the some . Thi s r ema rkable fact- i s bel i eved to be true only of light. Th e s peed of a bul l et, for exampl e , d epen ds i n port on t h e speed of the g u n from wh ich it i s fired and on the speed of the ob server as wel l . The speed of sound varies with the speed of the measu rer but not with the speed of the sourc e . The speed of l ight i s independent of both source a nd m ea surer. It i s a u niversal constant, one of the m ost i m portant constants i n a l l of sc ience . The constancy of the speed of light i s a ba sic postu late of E i nstei n ' s theory of rel a tivity . THE SPE E D

•

Approx imotely 3 X 1 01'( meters per second

7

carry energy in a l l d i rec­ tions through the universe. Al l objects receive, a bsorb, a nd rad iate these waves, wh ich can be pictured as electric and mag netic fields vi brati ng at right angles to each othe r a nd a l so to the di rection in wh ich the wave is t rave l i ng. Light is on e form of e l ectromagnetic wave . Al l electromagnetit waves tra vel i n space at the same speed-the speed of lig ht. Electromagnetic waves show a continuous ra nge of frequen cies and wa velengths ( pp. 1 0- 1 1 ). Frequ ency i s the num ber of wa ve crests pass i n g a poi n t i n one s econd. E lectromagnetic wa ve freq uenc ies run from a bout one per second to over a tri l l ion-tri l l ion ( 1 024) per second. For l ight, the freq uencies a re fou r to eight hundred tri l l ion ( 4-8 x 1 014) waves per second. The freq uency times the wavele ng th gives the speed of the wa ve. The higher the freq uency the shorter the wave l ength. ELECTROMAGNETIC WAVES

ELECTROMAGNETIC STRUCTURE OF LIGHT WAVES

magnetic fiel d

8

E l ectric a n d magnetic fields ar• a lways pe rpendicula r to eac other and to the d i rection o motion .

A "SNAPSHOT" OF A GREEN LIGHT WAVE

0

1 0,000 A

5 ,000 A

WA VELEN GT H is the di stance from the crest of one wav e to the crest of the next. The height of a wave crest is its ampli tude and

1 5 ,000 A

is re lated to the energ y wav e . T h e wave shown di ag ra m is a g reen l i g h t Di stances are i n Ang stroms

of the i n the wa ve . ( p. 6 ).

with wave lengths less than O 1 A, are pen etrati ng radiations that are absorbed ve ry l i ttle in pa s s i ng through so l id matter . Th e amount of th e a bsorpti o n d epends upon th e d e n s ity o f the mate­ ria l , so these ray s are u seful for maki ng shadowgram s (X-ray pictu re s ) of th e den ser parts of an ob ject. GAMMA RAYS AND X-RAYS,

ULTRAVIOLET RAYS are produced i n great quanti ty by th e sun and by spec ial ty pe s of l am p s . Thoug h not de­ tected by the eye, th ey do affect ph otog raph ic fi l m . Th ey a l so ca use sunta n . Ord i na ry glass do es not tran smit much u l traviolet so yo u d o not ta n b ehi nd an o rd i na ry win dow. The wave l e ng th s of u l travi o l et ray s ( 1 0 A to 3,5 00 A) a re longer than th ose of X-ray s bu t shorter than th ose of l i g ht . A bit longer tha n l i g ht are the wave s of infra red rad iation wh i c h we sense as h eat.

9

LIGHT WAVES I N T H E

frequ ency i n cycles per second

i ndu ction . heating

power

3

X

1017

1()8

1()4

102

1015

1013

1010 i nfrared rays

radio waves

1011

1012

109

107

wavelength i n Angstrom un its

is that portion of th e e l ectromagnetic spectru m that norma l l y stimulates t he sense of sight. Electromagnetic wa ves exh i b i t a continuous range of frequen cies a nd wa velengths. In the vi s i b l e part of the spectrum these frequencies a nd wa ve lengths a re what we see a s colors . The wavelength s of l ight ra nge from 3 , 5 00 A to 7,500 A. The wavelengths of i nf rared rays ( 7,500 A - 1 0,000,000 A), longer tha n l ig h t rays, are not detected by the eye, and do not apprec iably affect ord i na ry photog raphic film. They are a l so ca l l ed hea t or the rmal rays and give u s the sensa tion of warmth. Because all electromagnetic wa ves a re basica l l y a l i ke, we can expect them t o behave i n a s i m i la r m a n ­ n e r . Differences are rea l l y b u t a matter o f deg ree a n d a re due to t h e d ifferen ces in freq uency. Light waves a re u nique o n l y in thei r visu a l effects . The con cept of color, for in sta nce, ha s mea ning only i n reference to l i g ht waves.

VISIBLE LIGHT

10

106

ELECTROMAGNET I C SPECT R U M

1Ql4

1Q20

1Ql8

1 Ql6

1Q22

1Q24

LIGHT

yel low­ g reen

green

blue-green

THE ELECTROMAGNETIC SPE C­ TRUM, shown above, covers a l l known rad iation o f w h i ch l ig h t is a s m a l l b u t i mportant part.

The expa nded v i s i bl e portion of the el ectromagnetic s pectrum is seen as a continuous g radation i n hue from red through violet.

WAVELENGTH V I SI BLE R AYS

OF

The wavelengths ( .,\ ) of v i s i bl e l ight are as s ee what we colors. Red has the l ongest waves, v iolet the shorte st.

55

00

����������������

7000

1 1

.

.

�,.

�;.., .

..

. �-?r; ;'ib

J

4000 A

._;��

I

5 000 A

4 5 00 A

Spec trum of sunl ight s howing

( p. 5) sepa rate s a bea m of l ig ht into its component wa ve l engths and d i splays th em in a spectr u m . The spectrum of an i ncandescent l a m p is a con ti n uous ba nd of colors ra ng i ng from violet through bl u e, g reen , ye l low, orange, to red . Each color blends into its neig hbors i n a n un broken ba nd of wa ve l engths . Sol i d s , liqu ids , and gases at ve ry h i g h pressure g ive con ti n uous spectra i f they a re made hot e no ug h. Ga ses at low pres s u re have disc rete spectra consi sting of colored l i n es or bands with dark spaces between. A ga s gives a l i ne spectrum if its mo l ecu le con s i sts of a sing l e a tom, a ba nd spectru m if it consists of more th an one ato m . The gas can be identified by the pa ttern of its spe ctral l i n es or bands, a nd its tem pe ra tu re can be dete rm i ned by the i r relative i n tensities.

A SPECT ROGRAPH

TYPES O F SPECTRA

Conti nuous spectrum o f i n candescent lamp " "

'

:;-� .� :"' t...,· > .

"•

�.�:���

. \�:·',.

Line spectru m of barium

:

;

I

I

Band spectrum of carbon a rc in a i r

!

I

I'

: ,

!,

i· II

I

5 5 00 A

-·� '

'

'

�,:"�., ,. .

" .. '"·'

���t: . •.

6000 A

-

- �"'

.

•.

6 5 00 A

7000 A

principal F ra u n hofer l ines ( p . 2 1 ) .

If a l i n e o r narrow band of the spectrum i s i solated by a s l i t or by colored fi l ters, the l ight that comes through is ca l l ed monochromatic l i g h t . It con s i sts of a single wa ve l en gth or a very narrow range of wa vel engths a nd excites in the ob server a sensation o f color, such a s red, g reen , or b l u e . White l ight, such as s un l i ght, i s a m ixture o f a l l visible wavel ength s . An object that re­ flects a l l wave l engths equa l l y appears white to ou r eye s . Each l ight source e m i t s a cha racteri stic spectrum which can be plotted on a g ra ph showi ng how the re lative energy va ri e s with th e wa ve length. This relation­ ship i s ca l l ed th e spectra l energy d i str i bution c u rve for th e l i g ht source. Most so urces a l so give off invisible ultravio let a nd i n frared rad iation, so the comp lete spec­ tr um usu a l l y i n c l u des mo re th an j ust vi s i b l e l i g ht . SPECTRAL E N ERGY D ISTR I B U T I O N C U R V ES

2 00 1 60 >-. Ol

4; 1 2 0

c w

.�

4)

0

Qj

80

a::

40 0

4000 A

5000 A

6000 A

Wavelength ( in Angstroms )

7000 A

13

is a m easure of the ra te of random motion s o f mo l e­ c u l e s . Abso l ute zero i s th e tempera­ tu re a t wh ich all s uch m otion s a re at a theoretical mi n i m u m . The Kel­ vi n , or a bsol ute, te mperature sca le, wi dely used in scientific wo rk, sta rts at absol ute zero. Th e freez ing po i nt of wa ter i s 273 ° K, a nd th e bo i l i ng po int is 373 ° K. Eq uivalent tem pera­ tu res on th e Fa hren he it ( F) a nd Cel s i u s ( C ) tem pera tu re scales a re shown at left. Tempe ra tu re a nd the color of a hot object a re ofte n c l o sely related. As a piece of i ro n i s heated it changes in color from g ra y to red, to ora ng e, to ye l low, a nd fi n a l l y to wh ite . TE MPERATURE

F

c

K

is be ing cont i n u a l l y ex cha ng ed betwee n every object and its surro un d i n g s . The amount and qua l i ty of this radi ati o n d epends upon th e te mpera­ tu re a nd material of both th e em itter a nd the ab sorber. When two objects a re at a bout the same te mperature, _ l i tt le heat i s transferred between them . Whe n o ne object i s much hotter tha n th e other, heat fl ows to th e colder one. This occ u rs wh en you h old o ut you r ha nd a nd fee l th e warmth of a hot stove. Yo ur ha nd rad iates less en ergy tha n it recei ves . Th e rate at which an object em its th i s rad iant energy i s proportional to th e fo urth powe r of its Kelvin tempe ratu re . Do ubl i ng thi s tempe ra­ tu re i ncrea ses the ra te of ra diation 1 6 times.

TEMPERATURE SCALES

14

RADIATION

is a n a b so l utely b l a c k body. It a bsorbs a l l the ra diation that fa l l s on it. Every light source is a radiator, but some a re m o r e efficie nt tha n others . A n o bject that is a good a bsorber of radiation is a lso a good emitter. A s m a l l d ee p h o l e o r cavity in a g ra p hite block serves a s a practica l b l a c k body. Any light that enters the hol e i s reflected m a ny tim es fro m t h e wa l l s a n d is pa rtly a b­ sorbed at e a ch reflection u- n til no light remains. Th us the hol e a p pe a rs perfectly b l a c k . If the wa l l� of t h e cavity a re heated, however, they give off radiatio n in a l l directio n s . The ra diatio n -that esca pes fro m the hole is ca l l ed b l a c k-body radiatio n . T h e spectra l distribution of radia nt e n e rgy e mitted by a heated b l a c k body d ep e n d s o n ly on its Kelvi n t e m p er­ ature, and not a t a l l o n the materia l of which it is m a d e . At low te m perature (bel ow 800°K) o n ly infra red radia­ tio n results. At a bout 6,000°K (th e tem p e rature of the su n's surfa c e), the peak of the spectra l e n e rgy c u rve is nea r the mid point of the visi b l e spectrum. Both ultra­ vio l et and infra red radiation a lso o c c u r . THE BEST RADIATOR

The standard u n it of l ight­ sourc e i ntens ity, the cand le, i s 1 I 60 of the intensity of 1 cm2 of a black body at the tempera ture of melting platinum .

Btack Body Radiation EN ERGY DISTR I BUTION

1 0,000 A

O F A BLAC K B O DY

At a g iven tempera ture , there i s a spec ifi c curve that represents the energ y distribution of o black body. At higher tem pera­ 1 00 tures, the pea k of the curve >E> occurs at s horter wavelengths. 4) &5 5 0 4)

·�

0

�

0

may be a ss igned to any l ight source by m a tc h i ng i t vi s u a l l y aga i nst a b lack-bod y rad iator . The temperature at wh ich the black bod y matches t h e c o l o r of a light source i s s a i d to be the color tem pe ra ture of the sou rce . For incandescent sources, such as an ordi nary ho usehold l ight bu l b, the color tem perature is rel a ted simply to the true tem pe ra­ ture and i s often a pproxi matel y equ a l to it. An ob server sees the sta r Antares, with a col or tem pe ra tu re a l most 5 ,000 ° K, a s red . Sirius, at about 1 1 ,000 ° K, i s much hotter a nd appears wh i te . The color tem pe ra tu re of some l ight so urces, however, ha s nothing whatever to do with t he actu a l temperature. A ' ' daylight' ' fl uo re scent tube, for example, may have a ra ted color tem perature of 6 , 5 00 ° K a nd yet be so coo l that i t is not un com­ fortable to touc h .

COLOR TEMPERATURE

CO LOR-TEM PER ATU RE CLASS I F I CA T I O N OF STARS COLO R 25

B lue­ white

W h ite

Yell ow­ wh ite

Y e l lo w

8

A

F

G

,."""'··· • i� I

•'

"'

.

��·�··�!}� :�:· '

-----lf'��..

.

SPECT R AL EN ERGY CURVE Compare the sp ectral energy curve of a 40-watt "daylight"

100

iJ; 80

Qi �

60

fluorescent lam p with those of g;' 40 the average daylight curve ''§ and of a tungsten l amp curve

(� 22) .

�

20

0 Wavelength

4000A

6000A

27

o r the e m i s s ion o f l ig ht, may b e due to causes other than heat ( thermolum i nescenc e ) .

LUMINESCENCE,

C H EM I L U M I N ESCEN CE i s the emission of light during a chemi­ cal reaction. When a formalde­ hyde solution is m ixed into an alkaline alcohol solution, chemi­ cal energy i s changed into light, cau s i n g the mixture to g low. BIOLUM I N ESCEN CE is the pro­ duction of l i gh t by che m i l u m i n es­ ce nce i n l iv ing organ i sms, as in ce rtai n fu n g i, bacteria, comb j e l l y­ fish ( left), fireflies, and fishes. T h e Ra ilroad W o r m , a beetle larva , is biolum inescent in two co lors. FLUORESCENCE i s the produc­ tion of light when a s ubstance is exposed to u l trav iolet or oth er rad iation ( inc luding beams o f e lectron s o r o t h e r partic les ) . Most cases o f fluorescence are real ly examples of phosphorescence. PHOSPHORESCENCE is delayed fluorescence. The light emission continues for a time after the exciting radiation stops. T ele­ v i s ion tubes h ave a phosphores­ cent coating and thus produce pictures without a pparent flicker. E LECTROLU M I N ESCENCE offers a new sou rce of diffuse i l lumina­ tion for lighting. Alternating c u r­ rent appl ied to thin conducting pan e l s excite s l u m inescent mate­ rial sa ndwiched betwee n the m , producing a soft, easily regu lated g low.

28

ILLUMINATION I l l u m i nation i s often u sed as a general te rm that refers to the quantity and q ua l ity of l ight. The i l l u m i nation of a scene may be bright or d i m , harsh or soft, and perhaps even cold or wa rm . These terms refer loose ly to the amount, contrast, and hue ( color} of the l ig h t. In a nar­ rower sense, i l l um i nance i s the amount of l i ght received on a specified s u rface area . The range of sensitivity of your eyes to light i s so g reat that you are able to see c l ea r l y u nder widely different cond itions of i l l um inatio n . The ratio o f the i l l u m i nance at noonday to that on a moon l e s s night may be as great as ten m i l l ion to o n e . O n a c l ea r day there may be 2 0 times as much i l l u m i nation on the sunny side of a bu i ld i ng a s o n the shaded s i d e . Modern i ndoor l ighting for houses cal l s for i l l u m ination that i s a bout one fifth that fou nd on the shaded s ide of a b u i l d i n g on a clear day. Th i s i s 2 0 times more i l l u m i nation than was once cons idered adequate fo r homes . I m proved i l l u m ina­ tion g reatly inc reases the ease with which reading o r fine work c a n be done. The human eye can not d i stinguish the component wave­ length s of a l i g h t bea m , nor can it d etect s m a l l cha nges i n s pectra l d i stributio n . Neither i s the eye equa l l y sen s i ­ tive to a l l wave l ength s . Meas u rements made w i t h many people have prod uced a standard l u m i n o s ity c urve (p. 3 0 } that represents t h e relative sensitivity of the average eye to different wave lengths of light. Rad iant energy, i n c l uding l i g ht, i s a physical quanti ty that can be measu red d i rectly by severa l types of rad ia­ tion detectors, such as thermopiles, bolometers, and wave meters. Visible l ight can be measu red by a pho­ tometer or a l i g h t meter. 29

·;;; 0 .s: E

�

:::> ...J

0.8 0.6

Q) >

0.4

0::

0.2

Qj

'"6

-� ..,

0 ·S

§

""

�'b

�?§'

,o ..

Wavelength

0 4000 A

5000 A

LU M I NOSITY i s th e abil ity of l ig h t to exc ite th e sen sation of brig ht­ ne ss ( p . 32) . Th e standard l u m i -

6000 A

7000 A

nos ity curve peaks at 5 , 5 5 0 A, indicating th at our eyes are most sens itive to yel low-g reen l igh t . VAR I AT I O N S I N

Outd oo r fi l m , outdoor l igh t

AR T I FICI AL LI G H T i ndoor s i s very diff e rent from outdoor l ig ht, yet you rarely notice th a t y o u r green sweater i s of a somewh a t diff er­ ent color as you come indoo rs . Just a s your eyes adapt to ch ang e s in ligh t intensity, so th ey adapt to ch anges in l ig h t qual ity. Color ph otog ra ph s sh ow the difference because, u n l ike your eyes , th e fi lm ca nnot adapt its sen s itiv ity to th e ch a nged i ll umi nation . F i l m des i gned for indoor use with in­ ca ndescent lamps produces a very blu ish or cold picture if used outdoors. Film des i gned for out­ door use res u lts in a very orang e or wa rm picture wh en used with artificial light i n door s .

Outd oo r fi l m , indoor ligh t I ndoor film, ou tdoor l igh t

30

I nd oo r fi l m , indoor ligh t

+

Dawn

Midmorning

I L LU M I NAT I O N DAYLIGHT cha nges color co n ­ stantly from s u n rise t o s u n set. A co lor photog ra ph taken i n early mo rn i ng or in late afternoon wi ll have m uch warmer color s than one ta ken when the sun i s over­ head . Col or fi lm p roperly record s the diff erences in the co lo r of daylight, but human v i s i on sim­ ply compe n s ates for t h e diff er­ ence s . You not ice only extreme cha ng e s in the color of daylig h t, as at sunrise or s u n set.

M idafternoon

+

L ate a fternoon

i s a purely psychological concept. It i s a sensation of the observer a n d cannot b e measured by i n struments . The a b i l i ty of the eye to j udge absol ute va l u es of brig htness i s very poor due to its g reat powers of adaptation . The eye i s a very sensitive detector of brightness d i fferences, however, provided the two fields of view a re presented s i m u l taneously. The measurement of l ight by vi s u a l compari son i s the bas is of the science of photometry. Lig ht-source i n tens ity depends o n the tota l amount of l ight e m i tted and on the s ma l l ness of the con i ca l sol id ang l e i n wh ich i t i s emitted . Sta ted s i m p l y, i t is the amount of l ight emitted in a g iven d i rection . Brightness is a ssociated with the amount o f the l ight stim u l u s . It is the visual sensation corresponding to the perc eption of l uminance. L u m i na nce, the intens ity of l ight-sou rce per u ni t a rea, is a psychophysic a l property and can be m ea s u red. Lum inance a nd l ig ht- source in­ tens ity, often confu sed , are best de scribed by examples. BRIGHTNESS

T H E T W O LUM I N OUS SPHERES of d ifferent size ( be low left) have ide ntical light bulbs. They are diffuse emitters and so wil l ap­ pea r as luminous discs fro m any viewpoint. Both e m it th e sam e tota l a mount of light, and both hav e the same intensity . But the sma ller one wi ll appea r b righter to the ob server and wil l have the h igher l uminance.

In order for the two spheres to have the same luminance the larger sphere would require a larger ( more intense) light sou rce ( below right). It wou ld then emit the same amount of light per unit area of s urface as does the smaller sphere. The two spheres would then appear equally bright, but the larger one would have the g reater i n te n s ity.

T H E R ELAT I O N S H I P OF I N­ TEN S I T Y to con i ca l ang l e i s shown by co mparing a spot l ig ht and a floodl ig ht with identical light bulbs in reflectors of the same di ameter but dissim ilar shape (abov e ) . The spotlig h t bea m is concentrated into a smal l cone with rays almost pora l l el . The flood light, which e m its the same am ou nt of ligh t, s pread s it into a la rg e cone. The spotlight pro­ vides a m uch g reater i ll uminance but over a m uch smal ler a rea than the fl oodl ig h t .

I f w e look directly into the bea m of the light sou rce, the s potlig ht a ppears to be muc h brig h ter than the floodl ig ht . I n that d i rection on l y , the inten s i ty and the l u m i n ance of the spot­ light are g reate r than those of the flood light. If we l ook at the l ig h t source from other di rec tio ns out­ sid e the bea m, the inte n s i ty and lumina nce of the spot l ig h t are much less, because so much l ig ht is concentrated i n to th e beam that there is l ittle l eft to go in the se other directio n s .

are u sed i n the quantitative mea­ s u rement of i l l u m i nation. A few of th e more i m po rtant ones are g iven below for reference.

PHOTOMET RIC UNITS

LUM I NOUS FLUX i s the quan ­ tity of light. I t i s mea sured i n lumens. I nten sity is luminous flux per u n i t solid ang le. It is measured in ca ndl e s . ( 1 ca ndle 1 lumen pe r u n i t sol id ang le . ) I l l u m inance i s inc ident fl ux p e r u n it a rea . It i s mea sured in lux. ( 1 lux 1 l u men per m eter2 . ) l u m i ­ nance is intensity p e r u n i t area. It is m easu red in ca n­ dl es per meter2•

1

'

po i nt l ig h t . un1 t ource

=

=

o

1 eng I e I'd

''tJ.rr e, /tJ�

;":

o

s

+-

1 m

f ------�

1m

area

=

1

m2

point l ight

L ig ht th at i l l u minates a one-square-meter sur­ face at one meter wi l l cover four square me­ ters at two meters and spreads over nine square meters at th ree meters .

ra diates its energy u n i formly so th at l ig ht rays spread out from it i n a l l d i rectio ns. I l l u m i nation at a poi n t on a surface va ries w i th th e in­ te nsity and shape of th e l ig ht source and th e d i sta nce of th e su rface from i t . The amou nt of l i g ht fa l l ing on a u n i t a rea (the i l l u m inance) d ecrea ses with th e square of th e d i sta nce (the i nverse square law ) . A POI NT LIGHT SOURCE

SHADOWS a r e formed when l i g ht cannot pa ss through an opaq ue body i n its path . I l l u m ination of the area beh ind the body i s cut off. A sma l l ( p inpoint) or dis­ ta nt source of l ight casts a sharp shadow. A near, large, or diffuse l ight sou rce produ ces a fuzzy shadow with a centra l dark a rea that receives no l i g h t ( umbra) and a l i ght outer area ( penumbra ) which receives some l ight from part of the source.

- - -���� ---------'•'

point source

\II I -

�

�:�--;�ii.:�;:: d iffuse source

34

hand

------

--

--

i s a term u sed by an observer to d i stin­ guish between l ightness and darkness of colored ob­ jects, a s between l ight blue and dark blue pa i nt. It should not be confused with brightness (p. 32 ) . The observer's perception of l ightness is a l so a recogn i tion of a d i fference i n whiteness or g rayness between objects . I t i s a com pa rative term referring to the amount of d iffusely reflected l ight coming to the observer's eye from a surfa c e . The surface wi l l a ppea r white if it i s a good non-sel ective diffu se reflecto r a n d i s wel l i l l u m i na ted by white l ight. I f it i s a poor reflector or i f it receives l ittle or no i l l u m i nation, the su rface wi l l appear g ray, g rayer, or even black. B lack, then, i s the perception of an area from which the l ight i s i n s ufficient for deta i l ed vision . Wh ite i s the perception of a wel l - i l l u m i nated su rface whose reflectance i s high and non-sel ective . Gray i s the perception of a surface between these extremes .

LIGHTNESS

THE PE RCEPT I O N OF GRAYN ESS is influenced by, among othe r thing s, t h e illumination of the su rrou nd ing area. The gray area s at the bottom of the page are iden­ tical , but the one surrounded by

black appears to be much lighter than the one s u rrounded by white. Because o f the amount of light refl ected , the three areas be­ low a re seen as blac k , gray , and wh ite .

D

fa

absorbe nd scaUAre '1 lirJh' I ,."J I /

-

.,..,...,

These are the ways in which a light beam may behave. Not shown is polarization of light.

GLASS

LIGHT BE HAVIOR

light behavior i nc l udes tran s m i ssion, a bsorption, reflec­ tion, refraction, scattering, d i ffraction, interference and polarization, a l l of which are d i scu ssed in th is section . Transmission, a bsorption and reflection account for a l l t h e l i ght energy when l ight strikes a n o b j ect. I n the course of tra n s m iss ion, l ight may be scattered , refracted or po larized . It can a l so be pola ri zed by reflecti o n . The l ight that i s not transmitted or reflected i s a bsorbed and its energy contri butes to the heat energy of the mole­ c u l es of the absorbing materi a l . The modi fication of l ight through these processes i s respo n s i b l e for a l l that we see. W h e n a bea m of l ight strik es a thick sheet of g lass, part of the light may be r eflected, part abs orbed and scattered, and the remainder transmitted.

36

GA LILEO attem pted to mea sure th e s peed of l igh t but fai led . Ma n at B wa s to sh ow h is l i gh t wh en he saw l igh t from A. Man at A was to record elap sed ti me from fla sh of A' s l igh t to receipt of B ' s

l i gh t . Th i s simple ti me and dis­ tance meth od fai led beca u se th e ro und tri p travel time of on l y . 0000 1 1 ( 1 1 X 1 0 -6 ) sec. fo r a 1 -mi le separation was l es s th an h uma n rea ction time (0.4 sec . ) .

s o fast tha t for many yea rs scie ntists th ought that its speed wa s i nfin ite . The fi rst observa tions and mea s u rements wh ich gave a fi nite value to the s peed of l i g ht were made by the Da nish a stron omer O l af Roemer in 1 6 75 . Roemer was measuring the pe riod of revo l u tion of one of Jupite r's sate l l ites by timi n g its successive ec l i p ses be hind the pla net. H e fo und th at the m ea s ure­ ments made while th e earth was reced i n g from J u piter ga ve longe r period s th an th ose made wh i le the earth wa s a pp roach ing J u piter . Roemer conc l u ded th at the difference wa s due to th e fa ct that in the reced i ng posi­ tion th e light from each succe ssive e c l i pse had to trave l a greate r di stance to reac h th e ea rth a nd therefo re took a longer ti m e ( d iagram , p. 3 8 ) . He ca l c u l a ted th at l ight took about 2 2 m inutes to travel a distance equal to the dia mete r of the earth's orbit a bout the s u n . Roemer's meth od was correct but h i s accu racy wa s poo r . We now kn ow th at th e time requ i red i s n ea rly 1 6 . 6 7 m i n utes , or a bo ut 1 ,000 seconds. Since th e dia mete r of the ea rth's orbit is a bo ut 1 8 6 , 000,000 m i l e s , th e s pe ed of l ight is ca lculated to be a bout 1 8 6,000 m i les pe r secon d . LIGHT TRAVELS

37

R OEMER'S METHOD \

I

I I I I

Ea rth --

--

--

--

sate l l ite

--

-

Eclipse of J upite r ' s satel l ite was about 1 6 m i n utes late d u e to the added distance (diameter of earth ' s orbit) .

of th e speed of l ight a re made in th e laboratory. U s ing many diffe rent tech niques and much e laborate a pparatus/ scienti sts h ave m ea sured th e speed of l ight i n free space agai n and agai n , always striving for m ore acc u racy. The va l ue n ow a ccepted is 2 9 9 , 7 9 3 kilometers, o r 1 8 6, 2 8 2 m i les, pe r seco nd . The error IS bel ieved to be less th an o ne thousa nd th of one per cent. The p recise m ea surem ent of th e speed of l i g ht, a fundamental con sta nt, i s one of th e g reat technical achievements of ou r time . MOD ERN MEASUREMENTS

MICHElSO N ' S METHOD

l ight

octagonal

Miche lson, in 1 8 7 8 , reflected light from a rapidly rotating mi rror to a fixed distant m irror. By the time the reflected l ight retu rned, the rota ting m irror had moved enou gh so light was reflec ted at a d iff erent ang le, ena bl ing its speed to be ca l cu la ted .

38

i s of two kinds -diffuse and reg u l a r . Diffuse reflection i s the kind by which we ord i na ri l y see objects . It g ives us i n formation about their shape, size, color and texture . Regu l a,r reflection i s m i rrorl i ke . We don't see the su rface of the m i rror; i nstead, we see objects that are reflected in it. When l ight strikes a m i rror at an angle, it is reflected at the same angle. In d i ffuse reflec­ tion, l ight l eaves at many d i fferent angles. The deg ree of su rface roughness determines the proportion of d i ffuse and reg ular reflection that occurs. Reflection from a smooth, po l i shed su rface l i ke a m i rror i s mostly regu lar, wh i l e d iffuse reflection takes place at s u rfaces that are rough compared with the wavel ength of l ight. S i nce the wavelength of l ight is very sma l l ( about 5 ,000 A) , most reflection is diffuse. R E FLECTION

V I EW ED M I CROSCOPICALLY, a l l reflecti on is regu lar . The a ppear­ ance of d iffuse refl ection is due to the ma ny different angles that light rays encounter when they stri ke a rough surface . The re­ flection of each single ra y is reg ular- that is, i t is reflected at the same angle at which it

strikes the su rface. A fa irly smooth surface, such as that of a glossy vinyl raincoat, shows both d iffuse and regular reflection, the relative proportions depending on the angle of the incident light. But a rough su rface, such as that of a tweed coat, shows only diffuse reflec­ tion . I t has no " shiny" surface .

REGULAR REFLECTION

smooth surface

DIFFUSE REFLECTION

rough surface

39

with the type of mater i a l . Po l i shed metal reflects most of th e light that fa l l s on it, absor bs on ly a l ittle, a nd tran smits practica l l y no n e . Pa per is made up of partly tra nsparent fibers. Light striking paper may penetra te severa l fibers, being parfly reflected at ea c h s urface. The l ight th at finally reaches your eyes and lets you kn ow yo u a re looki ng a t paper ha s be en reflected a nd transm itted many ti mes. Al l th e re st of the l ight ha s been ab sorbed a nd added to the heat en ergy of th e molecules of th e paper . Most materi a l s a re q u i te selective i n the way they absorb and reflect the different wavelengths of l ight. A purple dye w i l l transmit blue and red l ight but wi l l absorb g reen l ight. Gold and copper meta l s reflect red and yel low wavelengths more strongly than blue. Si lver reflects a l l colors and therefore appears a l most wh ite . Meta l l ic reflection i s a n example o f pure s u rface color. Nearly all " o bject colors " are due to selective reflection and a bsorption of l ight. Obj ect colors are a n attri bute of the object, though the color seen at any time depends also o n th e color of the i l l u m i n ation . Tota l a bsorption of l ight m akes an object look black. REFLECT ION VA RIES

incident rays woven material

reflection revea ls the co lor and textu re of woven cloth . What we norma lly consider a s reflection i nvolves sel ecti ve ab­ sorption, selective reflection and refra c tion of light that pa rtia lly 40 penetrates the surfa ce .

r

-----------------

4) 1 V I

��

·;::; .!:

.,

'\�

I . .0 I E I g

.,

7:�

-----------------

I

.._

....,"'

..... 0

angle of incidence r

=

angle of reflection Reflecting

Surface

LAWS OF REFLECT I O N

1 . Ang l e of reflection equ a l s angle o f i n c idence. 2 . I ncident a n d reflected rays l ie i n the same p l a n e .

3 . I n c i d e n t and reflected rays are o n opposite s ides of

the normal -a line perpend i c u l a r to the reflecti ng surface a n d pa ssing through the point of i n c idence .

A PLA N E M I R RO R re­ ve rses a scene from left to rig h t. O bj e cts held i n the left hand of a su bject a ppear to be in the rig h t hand of the imag e . Al l ob­ jects seen in the mir­ ror a ppear to be as fo r be h ind the su rface they actual ly front of it.

A.

air

=

wavele ngth

air

glass

speed frequency x wavelength speed in air wavelength in air refractive index of glass speed in glass wavelength in glass =

=

is th e bendi ng of a l ight ray wh en it c rosses the bo undary between two d ifferen t mate ria l s , a s from a i r i n to water . Th i s change in d i rection i s d u e t o a cha nge i n speed . L i g h t trave l s fastest i n em pty space and s lows down upon entering matter. Its s peed in a i r is al most th e same as its s peed in s pace, but it trave l s o n l y 3/.1 as fa st in wate r a nd o n l y 2/J as fa st i n g lass. The refractive i nd ex o f a substa nce i s the ratio of th e speed of l ight i n space (or i n a i r ) to its speed in the substa nc e . Th i s ratio i s a l ways g reater than one. When a bea m of l ight enters a pane of g lass perpen ­ d icular to the surface ( a bove ), i t s l ows down , and its wavelength in the g l a s s beco mes shorte r in the same p ro portion . The frequency rema i n s the same. Coming out of the g l a ss, the l ight speeds u p agai n , the wave­ length retu rn i ng to its former s ize. When a l i g ht ray strikes the g lass a t some other a ng le, it cha nges d i rection as wel l as speed . I nside the g lass, th e ray bends toward the perpend i c u l a r or norma l . I f the two sides of the g l a s s are para l l e l , the l ight wi l l return to i ts orig inal d i rection when it l eaves the glass, even tho ugh it has been d i splaced in its passage . I f the two s ides of the g lass are not paral l e l , a s i n t h e c a s e of a p r i s m or a l e n s , t h e r a y emerg es i n a new d i r.ection . R E FRACTI ON

42

surfaces not para llel

surfaces parallel

LAWS O F REFRACTION

1 . I n c ident and refracted rays lie in the same p l a ne .

2 . When a ray of l i g ht passes a t a n a n g l e i nto a denser

mediu m , it i s bent towa rd the norm a l , hence the an­ g le of refra ction ( r) i s s ma l ler tha n the a ng l e o f in­ cide nce ( i ) , as be low. 3 . The i ndex of refraction of any med i u m i s th e ratio between th e speed o f l ig ht i n a vac u u m ( or in a i r ) a nd its speed i n th e med i um . THE I N D EX O F RE FRACT I O N (n) determines the amount of bending of a light ray as it crosses the boundary from air into the medium. For example, in any of the diaWATER

grams be low, the ratio between the line x and the line y, ( ) is equal to the refractive inde� ( n ) i f d i s the same length in both air and the medium. •

,

D I AM O N D

GLASS

.:.� _J

\ -t 1 I I

n

=

1. 33

n

=

1.5

n

=

�

o c

2 .4

43

AIR

FR OM A PO I N T SOURCE ( 0 ) under water, the refracted rays in air make larger and larger angles with the normal , as the angles of incidence become larger. At the same ti me, the a mount of ligh t reflected back i n to the water increa ses. Finally, for the ray 08, when the angle of refraction be-

l i gh t pas s i n g into a i r

comes 90°, a l l t h e light i s reflected. The angle of i n cidence OBA, ca lled the critical angle, for pure water is 4 9 ° ; fo r a ray striking a g la ss-ai r s u rface, it i s about 4 0 ° . Beca u se of this, a 45 -90- 4 5 ° pr is m reflects 1 00% of t h e li ght enteri ng it and can there fo re be u sed as a perfect mi rror ( p. 6 0 ) .

REFLECTIO N occu rs whenever a l ig ht ray str i kes th e surface of a med i u m wh ose refractive index is less tha n th at of th e med i um in which the l i g ht is travel ing. The a mount of l ight th at is reflected depends on th e angle at wh ich it hits the s u rface . light from a point sou rce ( above) h its the s u rface at ma ny a ng l es .

I NTER NAL

is the separation of light i nto its com­ ponent wavel ength s . One method of d i s persing a l ight beam i s to pass it through a g lass prism-a th ick piece of g lass with flat non-pa ra l lel sides ( below). The re­ fractive i ndex of a l l mater i a l s ( p . 4 3 ) depends s l ightly on the wave l ength of the l ight. Fo r glass and other tran sparent materi a l s the refractive i ndex i s l a rger fo r the short ( bl u e ) wavelengths than for the longer ( red ) ones. Thus, when a beam of white l ight is passed th rough a prism, the blue rays w i l l be bent more tha n the red rays -that is, the l ight s preads out to form a spectrum . The co lors i n the spectrum a ppea r i n the o rder of i n ­ crea s i ng wave length: vio l et, b l u e , green, ye l low, orange, a nd red . Sir I saac N ewton first exp l a i ned the s pectrum . H e showed that, co ntra ry to popu l a r bel ief, the prism d i d not create the beautiful colors, but o n l y made visi­ b l e the components of white l ight. Scienti sts make use of d i spersion i n the analysis of l i g ht em i tted o r a bso rbed by va rious materi a l s both o n t h e earth and on other bod ies i n s p a c e ( p. 6 3 ) . DISPERSION

WH ITE LIGHT ( below ) entering through a na rrow s l it at the left stri ke s the pri s m a t an acute angl e . T h e longer-wa'lele ngth red ray s are be n t less tha n the shorter­ wavele ngth blue, s o a pa rtial sepa ­ ra tion occ u rs in t h e glass. The be am then strikes the second sur­ face of the pr ism, again a t an a cu te

angle, and the rays a re once aga in re fracted. They leave the prism as divergent ray s of different wavelengths. A wh ite screen i s pl aced some d ista n ce from the prism and the di fferent colors appear on it. The spectrum co n­ sists of many i ma ges of the slit, each of di fferent color.

i s the bending of waves around a n ob­ stacle. It is easy to see th i s effect for water wave s . They b e n d around t h e corner of a s e a wa l l , or spread as they move out of a channel . Diffraction of l ight waves, however, i s harder to observe . I n fact, d i ffrac­ tion of l ight waves is so s l ight that it escaped notice for a long time. The amount of bending is p roportional to the s ize o f l ight waves-about one fi fty-thousandth of an inch ( 5 ,000 A)-so the bend i ng is a l ways very sma l l i ndeed . When l i g ht from a d i stant s treet l a m p i s viewed th ro ug h a wi ndow screen it fo rms a c ros s . The fou r sides of ea ch tiny screen hole act as the sides of a s l it and bend l ight in fou r d i rectio ns, prod u c i ng a cross made of fo ur p rongs of l ight. Anoth er way to see th e di ffrac­ tion of l i g ht waves is to look at a di stant l i ght bulb th ro ug h a very na rrow vertica l s l i t . Light from the bulb DIFFRACTI ON

A PATTER N O F WAVES wil l m ove outwa rd , forming concentric c ir­ cl es, if smal l pebbles are drop ped reg u la rly from a fixed po int into a qu iet pond . If a board is placed in the po th of these waves, they will be seen to be nd arou nd the edg es of the board, cau s ing an

46

inte re sting patte rn where the waves from the two edges of the board meet and cross each oth er. When an obstruction with a ver­ tical slit is p laced in the pond in the path of the wave s, the waves s pread out in circ les beyond the s l it.

bend s at both edges of the s l i t a nd a ppea rs to s p read out sideways, forming an elo ngated d i ffraction pattern in a direction perpendic u l a r to th e s l i t . l i g h t can be imagined a s waves whose fronts spread out in expa nd i ng concentric spheres around a source. Each point on a wave front can be thought of a s the source of a new d i sturbance. Each point can act a s a new l ight source with a new series of concentric wave fronts expanding outward from it. Po i nts a re i n fi n itely numerous on the su rface of a wave front a s it c rosses an open i n g . As new wave fro nts fa n o u t from e a c h poi n t of a sma l l opening, such as a pinhole or a na rrow s l i t, th ey re inforce each othe r when they a re i n p h a s e ( p. 4 8 ) a nd cancel ea ch other when they are comp l etely o ut of ph ase . Th us l ig hter a nd da rker a reas form th e banded diffrac­ tion patte rn s . DI FFRACT ION PATT ER N S are formed when light from a point source passes through pinho les and s l its. A pinhol e gives a c i rc u­ lar pattern and a sl it giv e s an elonga ted pattern . A sh arp image is not formed by l ight passing through beca use o f d iffract ion.

As the pinhole or s l it gets smaller, the diffraction pattern gets larger but di mmer. In the d iffraction patterns shown below the a lter­ nate l ight and dark s pa ces are due to interfe ren ce (p. 4 8 ) be­ tween waves arriving from d if­ ferent parts of the pinhole or slit.

CONSTRUCTIVE INTER FERENCE

1.

i n-phase waves

2 . i n - phase

waves combined

.:M= +f-1

DESTRUCTIVE I NTER FERENCE 1.

��;�:-pho•e

2 . out-of-phase waves combined

is an effect that occurs when two waves of equ a l frequency a re superimposed . Th i s o ften hap­ pens when l ight rays from a s i n g l e source travel by d i fferent paths to the same poi nt. I f, at the point of m eeti ng, the two waves are i n phase ( vi brati ng i n u n i ­ s o n , and t h e c rest of one coi nciding w i t h the crest of the other), they will com bine to form a new wave of the same frequency. The ampl itude of the new wave i s t h e sum of t h e a m p l itudes of t h e original waves. The process o f form ing th is new wave is ca l l ed construc­ tive i nterference . If the two waves meet out of phase ( crest of one co­ inciding with a trough of the other), the res u l t i s a wave whose a m p l itude i s the d ifference o f the orig i n a l ampl itudes . Th i s process is cal led destructive i nter­ ference. If the original waves have eq u a l ampl itudes, INTER FERE NCE

I NTERFER E N CE OCCUR S when l i gh t waves fro m a po int sourc e (a si ngl e s l it ) travel b y two d if­ ferent poths ( th rough the double

s i n gle s l it

slit). Their interference is shown by a pattern of a l ternate light and dark bands when a screen i s placed across t h e i r path. screen in

phase

l ight source

48

i n pha se

---

they may com pletely destroy each other, leaving no wave at a l l . Con structive i nterference res u l ts in a bright spot; destructive interference produces a dark spot. Partia l constructive o r destructive i nterference re­ s u l ts whenever the waves have a n i ntermed iate phase relation s h i p . I n terference of waves does not c reate or destroy l ight energy, but merely red i stributes it. Two waves i nterfere only i f thei r phase relationship does not change. They a re then said to be coherent. Light waves from two d i fferent sou rces do not inter­ fe re because radiations from differen t atom s are con ­ sta ntly changing th eir phase re lation s h i p s . Th ey a re non- coherent ( see lasers, p. 1 5 2 . ) wh ich change their appearance with the angle of viewing and the d i rection of the i l l u mination, are due to i n terference. The d e l i ca te hues of soap bubbles and oil fi l m s , the pale ti nts of mother­ of-pea r l , and the bri l l iant colors of a peacock ' s ta i l are a l l i ridescent colors . I RIDESC E NT COLORS,

A SOA P B U BBLE a ppears irides­ ce nt u nder white light when the th ickness of the bubble is of the order of a wavelength of l ig h t. Th i s oc curs because light wav es re­ flected from front and ba ck sur­ fa ces of the film travel d ifferent di stances. A difference in pha se results that may ca use destructive interference for some particular

wavelength , and the hue or color associated with that wavelength w i l l be absent from the reflected light. If the missing hue i s red, reflected light a ppears blue-green, the complement of red. If film thickness or direction of i l l umina­ tion changes, interference occurs at d ifferent wavelengths and the reflected light changes color. red reflections cancel if film thi ckness is one-hal f wavelength of red

white

light

source

white

saeen

When a beam of white light passes through milk, the blue compon ents are scattered, the reds are transmitted.

i s the random deflection of l ight rays by fine parti cles. When s u n l ight enters through a crack, scattering by dust pa rticles in the air makes the shaft of light v i s i b l e . Haze i s a resu l t of l ight scattering by fog and smoke particles. Reflecti on, diffraction, and i n terference all play a part i n the complex phenomenon of scattering. If the scattering pa rti cles a re of u n i form size and much sma l ler tha n the wavel ength of l ight, sel ective scattering may occur and the materi a l wi l l appear colored, as shown above . S horter wavel ength s w i l l be scattered much more strong ly than longer ones . I n genera l , scattered light wi l l appear bluish, wh i le the rema i n i n g d i rectly trans­ m i tted l ight wi l l lack the scattered blue rays and th us appea r orange or red . Many natura l blue tints a re due to sel ective scattering rather than to blue pigments . The blue of skies and ocean s i s due to th i s kind of scatter­ ing. B l ue eyes a re the resu l t of l ight scattering in the i r i s when a dark pigment is lacki n g . Scattering b y larger particles i s nonselective and produces wh ite . The whiteness of a b i rd ' s feather, of snow, and o f cloud s - a l l a re due to scattering by parti­ c les which, though sma l l , a re large compared to the wavelength of l ight. SCATTERING

50

ABSORPTION AN D TRANSMISSION BY O PTICAL M ATER I ALS u l tra - v is- infrared v iolet i ble

quartz

I

I

rock salt

l

S ince crown g lass and flint glass absorb ultrav iolet l ight, a quartz

I

to 1 45 ,000 A I

bulb is u sed for the light source of a sun l a mp.

of light a s it passes through matter re­ s u l ts in a decrease i n intensity. Abso rption, l i ke scatter­ ing ( p. 5 0 ) , may be general or selective. Selective ab­ sorption g ives the worl d most of the colors we see . Glass fi l ters which a bsorb pa rt of the vi s i b l e spectrum are used i n research and photog raphy. An a bsorption curve for a fi l ter ( below) shows the amount of l i g h t absorbed at a particu l a r wave l ength . A u n i t th ickness of the absorbing med i u m wi l l always absorb the same fraction of light from a bea m . I f the first m i l l i meter thickness o f a fi l ter absorbs V2 the l ight, the second m i l l i meter a bsorbs 1h th e rem a i n i ng l ight, o r 1/4 o f the tota l . Th e third m i l l i m eter ab sorbs 1h of th e V4 , so on ly Vs of the light is tra n s ­ mitted th rough three m i l l i mete rs o f fi l te r . S e e a l so p . 1 1 1 .

ABSOR PTIO N

ABSORPTION CURVE OF A GREEN PLASTI C F I LTER

A

20

.Q c:

0. 0 "'

-

/

60

periscope

A 4 5 - 90 - 4 5 DEGR EE PR ISM will reflect l ight rays by tota l intern a l reflection . When the light rays enter perpend icular to one of the short faces of the prism, they are reflected totally from the long face and depart at right angles to the other s hort face ( 1 ) . These pri s m s are more efficient than s i lvered m i rrors . Two such pri sms m ay be used i n peri s copes t o direct t h e light down the tube and into the eye­ piece ( 2 ) . The 4 5 -90-45 degree pri sm may a l so be turned so that the l ight rays enter and leave perpendicular to the long face ( 3 ) . B i noc ulars ( p . 7 6 ) use such prisms in this way. A DOVE PR ISM is a modification of the 4 5 - 90-4 5 pri s m . The 90 deg ree corner has been removed . The prism inte rchanges the pos ition of two parallel rays, as shown . I f the pri s m i s rotated a round the d i rection of the light, the two rays will rotate about one another at twice the angular s peed of the pri s m rotation . Dove prisms u sed in optical instruments to i nvert an image are ca l l ed erecting pri s m s .

A TRI PLE M I R R O R has the shape of a corner symmetrically sliced from a glass cube. Light enter­ ing toward the corner from .any direction is reflected back paral­ lel to the direction from which it came. Tripl e mi rrors are used in bicycle and other reflectors, Placed along roads, they reflect the head lights of cars, warning motorists of curves a nd other ch anges in driving conditions.

A

60-60-60

DEGREE

PRISM

is used most frequently for dis­ persing light into its component wavelengths. A new or unknown transparent material is often cut into this shape so its optical prop­ erties can be studied with a spec­ trometer _ ( below), an instrument designed to measure angles of refraction of light rays. In the simplest and most familiar type of spectrometer, light from an

l ight is reflected back toward source

outside source enters a narrow, adjustable slit in a collimator tube. I t then passes through a lens that renders the rays of light parallel. The rays of light are directed onto a prism that re­ fracts and disperses them in a spectrum that can be viewed through a telescope. The spec­ trograph (p. 5 ) is similar in struc­ ture but is equipped to photograph the spectra .

SPECTROMETER

3rd order

2nd order

1 st order

image

1 st order

2 nd order 3rd order

A d iffraction g rating prod uces several orders of the s pectru m . The central image is white; h ig her order spectra overlap.

may be used i n stead of prisms to d i sperse l ight. Gratings were first used by Joseph Fraunhofer in 1 8 1 9 to observe the spectrum of the sun. I n 1 8 8 2 Henry A. Rowland perfected a m ethod of producing gratings of exception a l l y high q u a l ity. The m odern version con sists of fi ne para l l e l l i n es (up to 30,000 to the i nc h ) ru led i n an a l u m i n u m coating on a pla ne or con cave gla ss su rface . Light wa ves d iffracted from these l i nes interfere so that a l l wa vel en gths but one a re canceled i n any particu lar d i recti o n . Different wa ve l engths l eave th e gra ti ng at differen t a ng les and fo rm a spectru m . Grating s can be u sed i n the u ltraviolet and infra red as We l l a s in the v i s i ble reg ion o f th e spec­ tr um; spec ia l grating s are u sed with X-rays . D i spersion by a prism is grea ter fo r short wa ve l ength s th an for longer one s . Th e d i spersion o f a g ra ting, how­ ever, is a l most i ndependent of the wa ve len gth . G ra ti ngs prod uce a no rma l spectru m in which equal d i sta nces co r­ re spond to equ al wave l ength i nterval s a n d may be su­ peri o r to a prism in dispersion and reso lution . Grati ngs prod uce more than one spectrum a t the same time. These occ u r as a series of ever-wider spectra o n either side of a b r i g h t centra l i m a g e . T h e fi r s t spec­ trum on each side is known a s a first-o rder s pectru m and i s due to i n terference by a series o f waves which are out of phase by one wave length . The second ( second order) s pectrum o n each side is twice as long (has twice the d i spersion ) as the first. Each i s formed by a series of waves out of phase by two wave l engths . The third order spectrum overlaps the second order spectrum and DIFFRACTIO N

GRATI NGS

has three ti m es the d i spersion of the first. H i g h e r orders a l so overlap thei r neighbors and are longer but d i mmer. Diffraction grati ngs, l i ke prisms (p. 60), are used in spectroscopes for dispersion of a beam of l ig h t . The largest i n stru ments are of the g rati ng type. A spectro­ g raph record s the spectrum photogra phica l l y or e l ectron­ ica l ly . A monoch romator, also either prism or grating, uses a slit to i solate a narrow portio n of the s pectrum for sc ientific study. These instruments a re used to study the ma ny properties of l ight sources, from candles to d i sta nt stars, to learn the kinds of a to m s and m o l ecu les of wh ich th ey a re made, plus such othe r featu res as tem peratu res, velocities, and energy state s . Most of what scientists kn ow about the structure of ato m s was l ea rned with spectrographs. The same is tru e about o u r know l edge of d i sta nt sta rs, ne bu lae a nd ga lax ies - th e i r te mpera­ tu re s , velocities and chem ical structu re s . A PASC HEN SPECTROGRAPH uses a concave di ffra ction g rating in a simple ma nner. The g ra ting, the photograp h ic plate, and the entra nce s l it a re all arranged around a c ircu l ar trac k. Light

passes through the s l it, strikes the grating, and forms spectra of various orders a l l in focus on the circle. The photograph i c p late is p laced on t h e track record desired wavelength s .

PASCH EN MOUNTI NG

3rd order spectrum

concave grating

2nd order spectrum

1 st order spectrum centra l i mage

63

A LENS forms a n image by refracti ng the l ight rays from an object. Curved g lass lenses were first used a s s i m p l e mag_n ifiers i n the 1 3th century, b u t it w a s not ti l l nearly i 600 that the microscope was devi sed , fo l ­ lowed b y t h e tel escope a decade or so later. Mi rrors, '' which form a n image by reflecti n g light rays, had a l ­ ready been known for several centu ries a n d were easier to understa n d . A lens, however, has a n advantage over a m i rror in that it perm its the observer to be on the op­ posite side fro m the i ncoming light.

SIMPLE POSITIVE LE NSES ( a l so known as converg ing lenses) are sing le pi eces of glass that are thicker at their centers than at their edges. Each s urface is a sec­ tion of a sphere, and a l ine through the two centers of cu rva­ ture (AB) is the optic axi s . light passing through a lens is bent toward the thicker part of the g la s s . light rays parallel to the optic axi s are bent by the lens so as to converge at the focal point ( F ) of the len s . S im i l arly, l ight coming from the opposite direction converges at a second focal point an equa l d istance on the opposite side o f the len s . T h e distance from the center o f t h e lens ( C ) t o the focal point ( F ) is the foca l length of the lens. 1 _ -+---;._ 1

-

focal l ength

All pos itive lenses a re thicker at thei r centers than at their edges. They range from very th i n lenses with su rfaces of little curvature to thick lenses that a re nearly spherical in shape. Most lenses i n general use are "thin" lenses. Best known and widely used as a s i mple magnifier is the double convex lens whose surfaces usually, but not always, have the same curvature. The plano-convex lens has one side flat, the other convex . The posi­ tive meniscus lens has one con­ vex surface and one concave s u rface. The convex s u rface has a sma ller radius of curvature than the concave surface. As a resu lt, the lens i s a lways thicker at its center than at its edg e . double convex

planoconvex

positive meniscus

--------�O P.ti c a xis

64

po sitive le ns

focal point eye

I I I I

point

v irtual image

j-focal length -4- focal leng th -i When an object is p laced between a pos itive lens and either focal point, an upright, enlarged i mage wi l l appear on the same side of

1

object d istance

i mage distance

focal

foca l

rea l

the lens but farther away than the object. This virtual image ( p. 5 8 ) can be seen only b y look ing at the object through the lens.

,, I f an object is placed beyond the focal point, its i mage will be rea l , inverted, and located on the opposite side of the lens. When the object i s more than

twi ce the focal length from the lens the image w i l l be smaller tha n the object . With the obj ect c loser than twice the foca l l ength , the i mage i s larger.

MAGN I FICATION i s the ratio of length of image to length of ob­ ject. I t equals the distance of the image divided by the di stance of the object from the lens. Hence, an image will be larger than the object o n ly if it i s farther from the len s . The shorter the foca l

leng th of a lens, o r the greater it s convex curvature, the greate r its mag nifying power . Th i s power, expressed i n d io pters , is the foca l length of a lens i n meters divi ded into 1 . A l en s with a foca l length o f 25 em . ( 114 m . ) has a magn i fy i ng p ower of 4 diop ters . ma gn ification

eye real i mage on J retina

'1E��:3

I I

I

magn ifier

=

.image di stance ob ject distance

v irtual i mage

'

obj ect

i mage negative lens

eye

�

The eq u iconcave ( o r do uble-concave ) lens is used as a dimini shing gl as s to see how illu strations will redu ce in pri nting. SIMPLE N EGAT IVE LEN SES (also cal led diverging lenses) a re thicker at the edges than at the center. A negative lens alone can­ not form a real image as a posi­ tive lens does. l ight passing through a negative lens parallel to the optic axis i s bent away from the axis . . The foca l point of the negative lens i s located

by extending these diverging rays backward until they cross the axis. The image formed by a diverging lens i s always v i rtual, upright, and smaller and closer to the lens than the object. N egative len ses a re used to re­ duce images ( below), to correct nearsig htedness ( p. 69) and to co n­ stru c t compou nd lenses ( p . 7 1 ) .

Three type s of negative lenses

negative meniscus

is a fai l u re of a lens or m i rror to form a perfect image. Two of the six most i m po rtant types of a berration a re spherica l and c h romati c . Spherica l aber­ ration is ca used when pa ra l l el l i g ht rays pa s s i ng th ro ug h a lens a t d i fferent di stances from the optic axis a r e not a l l focu sed at the same po i nt. A d i aphragm that de­ creases th e aperture of the lens wi l l e l i m i nate the outer ra ys and red u c e spherical aberra ti on . Chromatic a berra­ tio n is cau sed by th e fa ct th at a lens be nd s eac h col or, or wave l ength, to a different degree .

AN ABERRATION

66

--.f ---i----

lens diaphragm

Sph erical Aberrati on Co rrec ted

Sph erical Aber rati on SPH E R I C AL ABER RATION de­ pend s on curvature of th e l e n s . light ra ys passing th rough th e outer part of th e lens bend too sh arply to pass th rough th e focal po int and form a fu z zy i mage .

If th e sph e rica l aberration is to be k e pt to a m i n i m u m, th e radii of curva ture of th e lens s u rfaces sh ould be large co mpared to th e di ameter of th e l e n s . A d ia ph rag m limits the lens a perture .

a screen here shows the lea st b l u rred i ma ge

I I

white l ight

CH R OMATIC AB ER RATION re­ su lts from unwanted d ispersion of l ig ht ( p . 4 5 ) in a le n s , so th at d iff erent colors a re focused at s l ig htly different distance s . It prod uces a blu rring o f th e imag e in o ptica l i n stru m ents .

C h roma tic a be r ra tion ca n be corrected by com bi ning two or mo re s i mp le l enses made of d if­ fe rent k in d s of g la ss , so th a t th eir di s persions can c e l each oth e r . S uch lenses a re said to be a ch rom ati c .

all

to

colors come focus

a common

FARSIGHTED EYE

Eyeglasses to correct farsighted­ ness have positive l enses.

lens

obje ct at normal near point

aid a process cal led a ccom modation . The mu scles attached to th e l ens of the eye change the sha pe of the len s so tha t the images of o b j ec ts at di ffer­ en t d i sta nc es are brought to a fo cus on th e reti n a . The ra nge of thi s ac comm oda tion, in norm a l eyes ight, is from a " near point" at wh ich the eye can see an ob­ ject i n most detai l (a bout 1 0 i n ches ) to a far po i nt on th e hor izo n . F o r s o m e people, t h e n e a r point i s much farther away than 1 0 inches. They see d i stant objects c l early but a re unable to focus on objects at reading d i stance. Such person s are farsighted ( a bove ) . The defect i n their EY E GLASS ES

Visual accommodation declines with age.

68

N EARSIGHTED EYE

Eyeglasses to correct nea rsighted­ ness have negative lense s .

distant objed

lens

eyes may be corrected by wearing spectacles with posi­ tive ( converg i n g ) lenses. An object now held at the norm a l nea r point i s s e e n or read d i stinctl y . With a g e , t h e lens of the eye loses some flexi b i l i ty and the m u sc l es at­ tach ed to it lose some tone. Th i s loss of accom moda­ tion va ries among peopl e . I t can be corrected by g lasses, usua l ly g round so the u pper part of the lens gives a correction for d istant visio n . The lower segment of these bifocal g l asses is a s l ightly th icker, positive lens for c lose work or readi n g . A nears ighted person can s e e objects clearly at close range, but can not foc us on those at a d i sta n c e . Th i s difficulty i s remedied b y g l a sses with negative ( d iverg i n g ) lenses. With t h e m , d i stant objects a re s e e n as though they were with i n his range of accommodation . CONY ACT LENSES are a form of eyeg lasses that f1t directly over the cornea of the eye, float­ ing on the layer of tears that covers its s urface. Such small lenses correct severe refractive eye conditions and have special advantages for ath letes, actors, and others .

contac t len s

12 1 1 1 0� �2 3

9

·� �· 5

7

6

Asti gmati sm test chart . Rota te page a t a rm ' s l ength . Al l lines should a ppear equa lly inten se.

6

For some person s with severe astigmatism, the test chart might look like th i s .

of th e eye i s due to a fa u l t i n the cu rva­ tu re of th e cornea or lens ( page 8 5 ) . If either curved surface i s not sym metrical, ra ys i n different p l a n es wi l l not be focused a t th e same d i s tance be h i nd th e l en s . Thu s part of t h e i mage w i l l b e o u t of focu s . Th i s defect can u s u a l l y be corrected by wea ring spectacles with cyl i nd rica l lenses ( below) i nstead of spherical ones. For large corrections or i rreg u l a r corneas, contact len ses may be presc ri bed . Almost two out o f th ree persons have at least a m i l d form of astigmati s m . ASTIG MATISM

focus i n one plane ASTIGMATIC GLASSES have cy­ l in d ri ca l lenses, which a re c u rved about a transverse axis rathe r t h a n a round a point, as in s i mple pos itive lenses. Cylindrical lenses can aid i n correcting asti gma­ tism, i n which the lens o f the eye does not have sufficient c u rva­ ture around its vertical a x i s . \,

70

.... ...

---

are u sed in p re c i s ion optica l i n stru­ ments such as microscope s , te l escope s , a nd expen sive cameras. Th ey con s i st o f two or mo re s i m p l e l e nses (e lements ) com bined i n such a way th at the a be rration s are m i nim i zed. The e lemen ts are sometimes ceme nted to­ geth er a nd sometimes caref u l l y spaced a pa rt in the same mount. De sira bl e c ha racterist i c s of an optica l i mage a re brig htness, h i g h reso l ution o f fi n e d etai l , ed ge sha rpness o f th e i mage, flatne ss o f fie ld, and good contrast between l i g ht a nd dark porti o n s . A l l of th ese cannot be ac h i eved at the sam e ti m e, and even the be st l e n s is a compromise, achi evi n g one good qual ity pa rt ly at the expense of a noth e r . Comp uters are now used i n lens design to help choose the proper curvatu re s , m ater ia l s , and spa c i ng s of th e elements fo r best cor rection of th e a be rration s . Some i mage-form i ng i n s tru ments u s e both m i rrors a nd len ses . Such sys tem s, c onta i n i ng both re frac ti ng a nd reflecti n g e l em en ts, are ca l l ed catad ioptr i c . COMPOUND LENSES

eyepiec e MAG N I FY I N G EYEPIECE i s u sed in simple viewing and measuring in stru ments. One form i s the pocket doublet magnifier. Carefu l lens grinding and spacing reduces aberration s . TELE PHOTO LEN S for a camera is a compou nd l e n s which gives the effect of a long focal length system with a relatively short len s·to-film distance. M I CROSCOPE OBJECTIVE LEN S h a s a short foca l length a n d re la­ tively large aperture. Its h igh to magn ification emphasizes aberra- � tions , so that a h i g h order of 1!01E:-++-H-+---++-+--_., eye­ piece correction is requ i red.

Ray Diag ra m of an Opera G la s s

wa s in vented by a Du tch opti c i a n , Hans Lippershey, i n 1 608 , som e 3 00 years a fter the invention of spectac l e s . Gal i l eo received news of t h i s inve ntion in 1 6 0 9 , a nd without seei ng th e o rig ina l , he con structed a telescope con s i sti n g of o ne pos itive a nd one n egative len s mounted i n a d i scarded orga n p i pe . The same opti­ cal a rrangement is used m ore effic iently in opera gla sses, wh ich are usually b i nocular. The pos itive lens i s ca l l ed th e objective , and the n egative lens i s the o c u l a r, or eyep i ece . The field of view afforded by opera g l asses is rather sma l l at large mag ni fication s , so opera gla sses are u sua l ly of low power - 3 to 5 mag nifi cation s . S i nce th e eyepiece is a negative len s, the image seen by the viewe r is u pr ight. The spyg lass, or te rrestria l te l escope , p rovides a n up­ right i m ag e because a third lens l ies between th e ob­ jective and ocular ( be low ). Th i s make s th e i n str ument q u i te long; so, fo r co nven ience, it i s usually made col laps ible. TH E TELESCOPE

Ray d ia gram shows how i n verted image i s erected in a si mple spyglass .

72

erecting l enses

eyepiece (ocular)

IN THE REFRACT I N G TELE­ SCOPE the o bjective forms a real , inverted image of a dis­ tant object at the prime focus . The eyepiece then forms a magni-

fled virtual image. Magn ification depends on the abil ity of the objective to furnish enough light and a good i mage for high mag­ nification by the eyepiece .

REFRACTING TELESCOPES use pos i tive len ses for both ob j ective a nd eyep iec e. The real i nverted i mage pro­ du ced by th e objective is vi ewed , e n larg ed , th rough th e eyep iec e . Refra cti n g te lescopes a re l i m i ted to a ma ximum a pe rture of about one m eter bec a u se of th e difficulty of produ cing ve ry l a rge pi eces of flawl ess opti c a l -q u a l i ty g lass , and then grind i ng c oa xi a l curva­ tu re s on both s i des o f th e l en s . Al so, a s l en ses are made larger, the a mount of light ab sorbed by the g l a s s inc rea ses, a nd th e wei ght of th e lens causes it to sag , i n trod uc i n g optical er­ ro rs . Th i s type of te l e­ scope i s u sed to study celesti a l bod i e s . The refra cting tel escope a t Yerkes Observatory in Wiscons i n has an objective lens 40 i n ches in di­ ameter, the la rgest u s ed success­ fu l ly by a stro nomers . The tele­ sco pe has a theoretical magn ify­ ing power of 4 , 000 diameters .

The Hale telescope on Palomar Moun ta i n in Sou thern Ca lifornia has the largest m i rror of any op­ tical telescope . The disk of pyrex glass has a diameter of 200 i n . a n d i s 2 7 i n . thick . I t w e i g h s 1 5 ton s . The concave surface is coated with a l u m i n u m oxide. V iew here i s of the " cage " in which the astronomer makes his observatio n s .

use cu rved m i r rors instead of a n obj ective lens. A l l telescopes over 40 i n c h e s i n dia meter are of the reflecti ng type . Since only the surface of a l a rge m i rror needs to be optica l ly perfect, it can be braced from the back to prevent di stortion. A wa ffle pattern mol ded into the back reduces the we ight and m a i n ta i n s rigidity. The front su rface of the mi rro r is ca refu l ly ground and po l i shed to a pa rabo loid . I t is then coated with a very th i n a l um i n u m fl l m, wh ich ox i­ d izes into a highly reflecti ng and dura ble surface. A 2 00-inch reflector co l l ects four times as much l ight a s a 1 00-inch i n strument. It sees twice a s far into space, and thus su rveys a un iverse eight times a s large as that seen by the s m a l l e r telescope. REFLECTING TELESCOPES

I n the reflecting telescope, light from astronom ical objects is reflected from a curved m irror to a prime focus where an in­ ve rted rea l image is formed . I n

the Newto nian fo rm , shown be­ low, a fl at mi rror is p laced i n the bea m to refl ect the pr ime foc u s towa rd o n eyepiece a t t h e s i d e o f t h e instrument.

Roy D iagram of a Reflecting Telescope

�anal

reflecti ng m ir ror

prime focus

74

pol ished first surface mi rror

l ight from sta rs

spherical m irror

curvature

paraboloida l m i rror

rad ius of c u rvature

Light rays striking a spherica l m irror pa ra l l e l to the optic axis are not a l l reflected through the foca l point ( F ) . But in a parabo-

loidal mi rror, the changing cur­ vature re flects a l l rays through the foca l po int, and a clear, sharp image results.

is s i m i lar to that of spherical len ses ( p. 6 7 ) . Para l l e l l ight rays that str i ke the m i rror at ·d ifferent d i stances from th e center are not reflected through th e focal point. Si nce they do not m eet a t a s i n g l e poi n t, the res u lting image is fuzzy . The defect becomes more seriou s a s the d i ­ amete r of t h e m i rror I s m ade l a rger i n proportion t o the rad i u s of c u rvature. To avoid spherical aber­ ration , m i rrors a re made with concave pa ra bo l oidal su rface s . Th ese a re u sed in tel escopes and search l ights. SPH ERICAL ABE RRATION OF CONCAVE MI RRORS

SC H M I DT CAMERA i s a form of te lescope used to take pictures of large areas of the sky. Its field of view is wider than other reflectors because of a " correct­ ing" lens inside. Since the field of foc us is c u rved, this in strument cannot be used for " eye" ob­ servi ng. Shown is the 72 -inch Sch m idt Camera at Mount Pa lomar.

BINOCULARS a re used, l ike a sma l l te l escope, to view d i stant objects . Th ey emp loy an optica l system of len ses and prisms to prod uce an enlarged erect image. The ocular and the objective lenses provide the magnifica­ tion and i l l u m inati o n . Between them is a pa i r of 4 5 -90-45 deg ree pr isms (p. 6 0 ) so arranged tha t the light pa ssing th ro ug h th e b i nocula rs i s interna l l y refl ected fo ur times, making the image erect . THREE F ACTORS are invol ved in the usefu l ness of bi noculars - m ag · nifica tio n , field of v iew, and l ight­ gatheri ng power . Magn ification must be suited to the purpose. Any movement by either the ob­ server or the observed is magn ified at h igh power. For hand-held uses, binoculars with magnifications of 6x to 8x are best. H igher mag­ n ifications req u i re a tripod or other support. Field of view is largely determ ined by the ocular lenses. The d iameter of the ob­ jective lens determ ines the l ight-

B I NOCULARS

76

gathering power-the larger the better if binoculars a re used at n i ght or i n shady woods . Binocu­ lars have central or individua l focus ( centra l preferred ) . They ra nge in mag n ifi cat ion from about 2 to 2 0 . Each binoc u la r has a n id entification ma rk such as 8 X 30 or 7 X 50. The first number is the mag n ifi ca tion , t h e second t h e objective d ia meter in m i l l imeters . An 8 X 30 g la ss has s l i ghtly g reater m ag n ifi cation but d isti nctly less light-gathering power than a 7 X 50.

MICROSCOPES, PROJECTORS AND ENLARGERS are s i m i l a r i n principle, but they differ i n pu rpose and de­ sign . In each, a positive lens forms a rea l image of a brightly i l l u m i nated object. With projectors, the i mage i s caught o n a screen; with microscopes, it i s viewed through an eyepiece; and with photog raphic e n l a rgers, the i m age i s p ro jected on l ight sen sitive paper, where it i s reco rded i n semi-permanent form . MICROSCOPES need intense il­ lumi nation of the object because the i mage is much larger than the object and the same amount of l ight must be spread over a large arec . The i l l u m i nation i s provided either by a tungsten lamp bulb or by an arc lamp, its light concen­ trated on the object by a lens or by a concave mirror used as a condenser. The microscope ' s ob­ jective lens has a short focal length ( from 1 em to less than 1 mm ) and prod uces a sharp image of a very smal l field .

THE MAG N I F ICATION a ch i eved with an optical mi croscope is ap­ prox i ma tely equal to the power of the objective m u ltipl ied by the power of the ocu l ar or eyepiece, prod ucing a n ov era ll magnifi ca­ tion of up to about 1 , 5 00 di­ ameters .

IMAGE SHARPNESS depe nds on re solving power rather than on magn i fying power, and is l imited by d i ffract ion effect s that blur the image. Two po ints sepa rated by a di stance less than one half the wavelength of light cannot be re so lved op tica l ly and w i l l appea r as one p o i n t i n s tead of two. With the usual i l lumina­ tion, this separation is approx i ­ mately 1 I 1 00,000 of a n inch . THE ELECTRON M I CR OSCOPE, by using a b ea m of electro n s with an effective wavele ngth much shorte r than that of l ig h t, obtains over 1 00 times the re solving power o f opti ca l m i c rosco pe s .

virtual image

PROJ ECTORS use high-i ntens ity tungsten or arc lamps for i l l umi­ nation . Two condensing lenses con­ centrate the l ight rays through the object ( u sually a film s l ide ). The converging rays pass on through the projection lenses, and the enlarged image is th rown on a screen. This is an inverted image, so s l ides are inserted upside down in a projector. With any given combination of lenses, the fa rther the image is projected the larger it w i l l be , and greater lamp inten sity will be requ i red. With some projectors ( below ) , an opaque object can be reflected on the screen . OPAQ U E PROJECTOR

PHOTOGR A P H I C E N LARGERS a re prec i s ion projectors with ad­ j u stments for foc using the image and contro l l i ng image size and brightness. Good enlargers pro­ vide un iform i l l u mination, a good ' lens system, and a rigid mount. I n operation, light from a lamp i� concentrated by a paraboloidal reflector, passes through a dif­ fusing glass ( or a conden sing len s ) , con tinues through the negative and then through the projection lens, which forms an enlarged i mage of the negative on the ea sel. I mage sharpness is adju sted by moving the projec­ tion lens relative to the negative. E N LARGER

Path of light in a pinhole camera

A pinhole camera photograph

CAMERA comes from the Lati n phrase cam era obscura, or dark cham ber, fo r a l l pictu re-taking i nstruments have a dark chamber to protect the sens itive fl l m from light. The simplest camera i s a l ight-proof box with a pinhole in one end and a piece of fl l m on the opposite inside wa l l . Light reaches the fl l m only when the pinhole is uncovered, usually fo r a few secon d s . U se o f a l ens instead o f a pinhol e a l lows m uc h m ore light to pa ss th rough, and the same pi ctu re can be ta ken in much sho rter time . If th e area of the lens is 1 ,000 ti m es as l a rge as the pi nh ole, th e pi ctu re which requ i red te n sec ond s can be made i n 1 I 1 00 sec ond , an average speed i n many mode rn cameras . I n ad dition to a l en s a nd fllm, a ca mera u sua l ly ha s a shutter, an adj usta b l e di aphrag m, a nd some ty pe of fo cusing ad j ustment. The sh utte r preve nts l i g ht from strik­ ing th e fl l m except when a pictu re i s be i ng made. A mec ha n i s m op ens the shutter and closes it automatica l l y afte r a length o f time . Cam era s h u tters m ay have speed s from 1 0 seconds to 1 I 1 , 000 of a secon d . Some cam eras take pictures at a m i l l ionth of a secon d, en­ ab l i ng man to see the un seeab le. The d iaphrag m can be ad j usted to ad mit va ry i ng amou nts of l i g h t each time th e shutter is open . Th e fo cusing mecha n i s m m oves the lens ba ck a nd fo rth to ach ieve a clear i mage .

79

THE SH UTTER of a camera a l lows light from the subject to enter the camera and strike the film for a time. Shfltters a re located either at the lens or at the film. Lens shutters are usually placed between or just behind the elements of the lens and usually have a set of leaves that snap open for the des ired time and then snap shut. Foca l plane shutters are next to the film and resem bl e a window blind with slots cut in it. A modern ver s ion has two curtai n s . As one moves and uncovers the fi l m , the second fol lows so closely that the open­ ing between them i s a mere slot. Exposure time is va ried by ad justing the wi dth of the slot.

THE f- N U MBER of a lens refers to the ratio of its diameter (d) to its focal length (f). For example, f I 8 refers to an aperture whose diameter i s 1/s of the focal length . A camera ' s lowest f­ nu mber corresponds to its wide­ open diaphragm and i s called the speed of the len s . U s ing a lower f-n u mber shorte n s the required exposure time, but reduces the depth of field ( p. 8 1 ). The dia­ phragm is set to the h ighest f­ n u m ber that can be u sed with a g iven shutter s peed in order to get maxi mum depth of field . Many modern camera s have a built-in photoelectric cell that selects auto­ matically the lens opening for the shutter s peed u sed.

The relation of the dicllltMI>ter lens to its foc al number. For exc:anJH;

f/3.5

the focal times jt,l �I!Mttt;t. ;.�;...;

FOCUS i s achieved when l ight from a n object pas ses th ro ug h a cam era len s and for ms a c lea r, acc urate i mage on th e fil m or viewer . l ight ra ys from an o bject po int diverge s l ightly as th ey ente r a camera . With a wide­ open len s , the rays pass th rough a l l pa rts of the lens and converge to th e i mage poi nt {below) . To foc u s the camera, the d i sta nce fro m th e lens to th e fi l m is ad j usted so that the point of co nvergence wi l l l i e at the fi l m s u r­ fa ce- not be fo re or be h i n d it. I t i s i m pos s i b l e to focus all po ints of a three-d i mens i onal scene on the film at th e same ti m e, but a satisfactory sharpness can usua l ly be ac hieved ove r a con sidera bl e de pth of th e scene. Adj u stment i s most critical when foc u s i ng on a nearby ob ject wi th a wide-open len s . It is l east c r i tica l for a di stant object with the sma l lest len s open i ng . The ac­ cu racy of th e foc u s i ng may be j u dg ed by o bserv i ng th e sharpness of the i mage on a tran s l ucent g l a s s su rface in reflex-type cameras or by super i m pos i n g two images in a ra ng efi nder in some other camera s . DEPTH O F FIELD i s the depth of a scene that is i n focus on the camera ' s fi l m . A large lens aper­ ture allows on l y a limited depth to be in focu s . The rest of the

scene i s fuzzy. With a smaller open ing, the cone of l i ght rays from lens to film converges and diverges less. Both near and dis­ tant objects a re i n better focus.

c·

obj ect points

Ot and a re added togethe r in phase . LASERS

Penc il-thin laser beam is u sed in optical and mechanical a lignment.

The l i g ht fro m a sma l l laser, i n fact, can be focu sed to form an i mage of l u m i n ance grea te r th a n th at of the sun ' s s u rfac e . The concentration of en ergy is so g reat th at extreme ly h igh te m peratures are p roduced . l i g ht ra ys fro m a laser can be beamed throu gh space w i th a fraction of a n inch sprea d per m i l e . The l ight i s a l so extrem ely pu re i n color ( m on oc h romati c ) . I N A RU BY LASER ( be l ow ) a ru by crysta l rod has plane parol· lei po l i s h ed ends which are silvered l i ke mi rror s . One end is on ly pa rti al ly si lvered and acts as a wi ndow for the light to get ou t. E nergy is su pplied to the ru by crystal by a powerf u l Aa sh tube lamp. T h is serves to pump the ( c h romium) atom s of the crys­ tal to a " meta stable" energy state in which they l inger fo r a few thousa ndths of a second before dropping to the g round state with the emission of a photon of l i g h t . M o s t o f t h e s e photpns p a s s ou t o f the crystal wa lls a n d a re lost, bu t soon one photon wil l move direc tly a l ong the rod and is re­ Aected from the polished end s, pa ssing back and fo rth a long the rod unti l it encounters a n a tom in the exci ted metastable state . The exc ited atom then rad ia tes

co ol ant

its photon in exact pha se with the photon which stru c k it. This second photon may in turn stim u­ late another atom , and t h i s " ca s­ code " process continues until the whole c rysta l i s fi l l ed with in­ phose radiation osci llati ng back and forth ins ide the rod . Pa rt of th is radiation is em itted thro ugh the ha lf-s ilvered end of the rod and becomes the laser bea m . Al l of th i s tok es place with in a few bi ll ionth s of a second, then the Aa sh tube fires ag ain and the process i s repeated . Modern la sers have been made of solid c rysta ls, glass, l iq u id s and ga ses. S om e operate i n pu lses as described, but ma ny emit co n­ tinuo usly. In a l l , the ra d i ation i s monoc h ro matic a n d coherent. I t i s this h i gh deg ree of coherence that makes laser l ig h t d ifferent from that of a ll other sou rce s .

co olant

1 53

HOLOGRAPHY i s a special kind of photography i n wh ich th e film c a ptu res not th e i ma ge of th e subjec t but the pa tter n of th e wavefronts of l ight reflected fro m th e subject. Invented by De nni s Gabor i n 1 9 4 8 , the pro­ cess took on n ew i m porta n c e when the l a ser wa s i n­ ve nted . I n addi tion to its u ses i n th e ente rta i n ment field, holography has many scientific and i nd ustria l appl ications, wh ich a re be i ng deve loped ra pid ly. T O MAKE A H O LOGRAM, a co herent l i gh t beam ( from a laser) is spl it i nto two pa rts-an object beam w h i ch i l l u mi nates the su b.· ject, and a reference beam wh ich is di rected to the photograp h ic fi l m by m irrors. At the fil m the I ight reflected from the sub­ ject interferes with the reference bea m to cau se a complex pattern of l i ght and da rk fringes in the film when it is deve l oped. W hen a ho logram i s h e l d in a beam of coherent l ig ht, part of

t h e di ffra cted light i s a rep ro· duction of the orig inal l ig h t wav e pa ttern t h a t came from the subj ect . T h u s , a v iewer look ing th rough the holog ra m can see a li fe-like reproduction of the o rigi· nal scene. Th i s v irtual i mage i s tru ly three-d imen s ional , s i nce the vi ewer can move his head and see a different pe rspective of the subj ect. Bes ides th e v irtual image, there is a lso a rea l i mage formed by the rays d iffrac ted i n another directio n .

laser beam

AR RA N G E M E NT F O R PRO D U CI NG A H O LO G RAM

USE OF H O L O G RA M I N PRO D U CI N G R EAL AN D VI RT U A L I MA G E S

rea l i ma ge

FI B E R OPTICS

i s th� res u l t of an i ngen ious a p p l i cation of a simple principle. I magine a l i g h t ray that has en­ tered the end of a s l i m so l i d glass rod . I f it a lways strikes the surface of the glass at angles greater than the critical a ng le (p. 44 ) , the ray wi l l be tota l l y reflected each tim e and trapped with i n the g l a s s . Reflected from side to side, the l ight ray wi l l be conducted a long the rod like water i n a hose. Final ly, the l igh-t ray hits the end of the rod at a s ma l l angle to the perpend i c u l a r and light ray traveling through fiber

A big advance i n fi ber optics came with the development of very fine clear fibers encased in a th i n coating of l ower refractive index. Total reflection takes place between the fiber and its coating. The fibers a re so thin ( a few micron s in diameter) that they a re flexible, and their coat­ ings al low them to be bou nd into bundles without i nterfering with each other's action . Such a bun­ dle can conduct light for severa l feet. When the fibers are lined up so they have the same re l ative pos ition at each end of the bun­ d le, they can transmit i mage s . This m a k e s pos s i ble a sort o f super-periscope so flexible that physicians can use it to examine the interior of body organ s . A sheath of unaligned fibers a round the bundle can carry i l lumi nation to the area being examined .

1 55

i s able to exit. Th i s explai ns th e success of bent l ucite and g l a s s rods a s l i ght-conductors in a dvertis ing d isplays and in simple i l l umi nati ng devi ces . A bu nd le of fibers prod uces a gra i ny ima ge l i ke a ha l ftone pr i nting proc ess. By drawi ng out the fibers so th at o ne end of the bundl e is sma l l er tha n th e other, the ima ge can be enlarg ed or reduced and its bri ghtness changed . The quality of the image can be improved by sha pi n g the end of th e bu nd le or by a d j u sti ng th e al ign­ ment of the fibers to elimi nate di sto rtions. M O R E I N FORMAT I ON

Burnham, Hanes, Bartleson, COLOR: A G U I DE TO BAS I C FACTS AND CON CEPTS, New York, N . Y . : John W ile y & Son s, 1 9 6 3 . A com ­ pact handbook of defi n itions and facts about color. Comm ittee on Colorimetry , Optical Society of A merica, THE S C I E NC E O F CO LO R, New York, N . Y . : Thomas Y . Crowell Co . , 1 9 5 3 . A complete authori tative report for the serious student, with extensive reference and glossary. Eastman Koda k, COLOR AS SEEN AN D P H OTOG RAP HED, Roche ster, N . Y . : A Koda k Col or D ata Book, 1 9 6 6 . A brief explanation of color principles and color films. Evans, Ra lph M . , A N I NT RO D U CT I O N TO COLOR, New York , N . Y . : J o h n W i l ey & Sons, 1 94 8 . A carefu l explanation o f all aspects of color in non·technica l l ang uage. Eva ns, Ral ph M., EYE, F I LM, AN D CAMERA I N C OL O R PHOTOG­ RAP H Y , N ew York, N . Y . : John W iley & Sons, 1 9 5 9 . A thorough discussion of vi sual perception and its relation to photographic representation . Fowles, G rant R. , I N TROD UCTI ON TO MODERN OPT I C S , New York, N . Y . : H olt, Rinehart & Win ston , 1 96 8 . A c ol lege textbook that emphasizes the latest dev elopments in o ptics, i n cluding lasers. Mann, Ida, and A. Pirie, TH E SC I E N CE OF SEE I NG , Ba ltimore, Md . : Pel ican Books, 1 9 50. All about eyes and how they work. Minnaert, M., THE N AT U R E OF L I G H T AND C O L O U R IN THE OPEN A I R, N ew York, N . Y . : Dover Publications, I n c . , 1 9 5 4 . Explanations of shadows, mirages, rainbows and similar phenomena . Neb lette, C. B . , PHOTOGRAPHY, ITS MATERIALS A N D PROCESSES, 6th Edi tion , Van Nostran d-Reinhold Book s, New York, N . Y . : 1 96 2 . An authoritative di scu ssion of a ll phases of photograph ic technology. Sears, F rancis W., O PTICS, 3 rd Edition, Read ing, Ma ss . : Addison­ Wesley, 1 9 4 9 . A popula r col lege textbook with excellent i ll u strations.

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-

I N DEX

Bol dface n u mera l s i n d icate m a i n covera g e . Aberra t i o n , 6 7 c h r o m a t i c , 66, 6 7 , 7 1 sphe r i ca l , 6 7 , 7 1 , 7 5 Absolute temperature sca l e , 1 4 Absorpt i o n of l i g h t , 36, 40, 5 1 , 57, 99, 1 1 0, 1 39 Acco m m o d a t i o n , 68, 69, 84 A c h r o m a t i c l e n s , 66 Ac h ro m a t i c l i ght , 1 05 1 07, 1 32 Ad d i t i v e c o l o r m i x i n g , 1 06, 1 07-1 08, 1 09, 1 1 2, 1 30, 1 3 1 , 1 32, 1 39, 1 45, 1 50 Ad vert i s i n g , i n f l u e n c e of c o l o r , 5, 1 43 Afte r i m a ges, 1 23, 1 24 Ames' d i storted room, 96 A m p l itude, 9 And romeda, 7 Ang l e of i n c i d e n ce, 44 Ang l e of refra c t i o n , 44 Angstroms, 6 Anoma l o u s t r i c h ro m a t i s m , 1 1 3, 1 1 4 Anscochrome f i l m , 1 46 Aqueous h u mor, 8 4 Arc l a mp, 2 6 , 7 7 , 7 8 Ast i g m a t i s m , 70 Astro n o m i c a l t e l e scope, 73 Atmosphere, 2 1 Auxochrome, 1 4 1 Ba r i u m , l i ne spectru m , 12 B i foca l g l a sses, 69 B i n o cu l a r cues, 95 B i nocu l a r v i s i o n , 95 B i nocu l a rs, 60, 72, 76 B i o l u m i nescence, 28 B l a c k body, 1 5 , 1 6- 1 8 B l i nd spot, 88 B l i nd ness, 88 c ol or , 1 1 3- 1 1 7 B o l o meter, 29 Brewster's a n g l e , 5 7 B r i g htness, 32-33, 9 1 , 93, 1 00, 1 0 1 , 1 03 , 1 20, 1 33 B u n s e n b u rner, 1 9 C a m e ra , 7 1 , 75, 79-8 1 , 92, 95, 1 47

Cand l e m e a s u re, 1 5, 33 C a r b o n a rc l a m p, 1 8, 1 9 Carbon a rc, band spectrum of, 1 2 Carbon b l ock, 1 9 Ca rbon f i l a ment l a m p , 22 Catad i o p t r i c ·instruments, 71 C e l s i u s tempera t u re 1 4 C h e m i c a l energy, 1 8 C h e m i l u m i nescence, 28 C h r o m a t i c a berra t i o n , 66, 67 C h ro mo gens, 1 4 1 C h romop hores, 1 4 1 C h romosphere, 2 1 C I E system, 1 1 7, 1 25, 1 28, 1 30-1 32 c h r o m a t i c i t y d i a g ra m , 1 3 1 , 1 32 Coherent l i g h t waves, 49, 1 52, 1 54 C o l l i m ator, 6 1 Color ad vert i s i n g , 5 , 1 43 b l i nd ness, 1 1 3- 1 1 7 b r i g htness, 1 00, 1 03 b u s i ness a n d i n d u s t r y , 1 42 cha rts, 1 26, 1 27 c h r o m a , 1 26, 1 27 cod es, 1 42 consta n cy, 1 20, 1 33 contra st, 1 2 1 -1 22, 1 24, 1 33 contro l , 1 42 d i ct i o n a ry of, 1 25, 1 28, 1 29 e m ot i o n a l effects of, 1 33, 1 36 harmony and d is­ cord , 1 34-1 35 i m ages, 1 44, 1 45 match i n g , 83, 1 1 2, 1 1 3, 1 25, 1 30, 1 42 m i xtures 1 00, 1 06-1 1 1 , 1 1 2, 1 39 monochromatic, 1 02, 1 04, mosa i c f u s i o n , 1 09, 1 51 n a t u re of, 98-1 1 7 n a t u re of l i g h t a n d , 4- 1 7 n e g a t i v e , 1 44, 1 45 of l i g ht, 98 packa g i n g , 1 43

pa i nts, 1 06, 1 1 0, 1 37-1 39 pe rcept i o n , 99, 1 1 8- 1 24 photogra p h y , 1 44 - 1 47 pos i t i ve , 1 44 p r i m a r y , 1 04 psycho l o g i c a l response, 98, 1 00, 1 26 psych o p h y s i c a l con cept, 9 8 , 1 0 1 , 1 28 p u r i ty , 1 02, 1 28 reprod u c t i o n , 1 45 sca les, 1 03 s e n sa t i o n , 1 00, 1 0 1 , 1 02 sepa ra t i o n , 1 45, 1 46 s ta n d a rds, 1 1 2, 1 1 7 s y m bo l i s m of, 1 33, 1 36, 1 43 syste m s , 1 25 - 1 32 t e l e v i s i o n , 1 09, 1 50- 1 5 1 temperature, 1 4, 1 6, 17 t i n t s, 1 34 tones, 1 34 tra n s m i s s i o n , 1 1 0, 111 t r a n s p a r e n c y , 1 45, 1 46 v a l u e , 1 26 C o l o r b l i nd ness, 1 1 3-1 1 7 C o l o r p h otogra p h y , 1 44- 1 47 C o l o r te l e v i s i o n , 1 09, 1 50-1 5 1 C o l o ra n t, 1 1 0, 1 1 1 , 1 40 C o l o red afte r i m a ge , 1 23 , 1 24 C o l o red f i l te r 1 1 0, 1 1 1 , 1 45 C o l o r i meters, 83, 1 1 2, 1 30 C o m m i ss i o n l n ter­ n a t i o n a l e de I ' E c l a i ra g e (CI E ) , see I n tern a t i o na l Commission on I l l u m ination C o m p l e m e n t a ry h u e s , 1 05, 1 07, 1 3 2, 1 34, 1 35 C o m p o u n d l en s e s 7 1 C o m p o u n d m i c roscope, 77

1 57

Cones, eye, 86-88 , 1 1 3 , 118 C o n structive interference, 48-49 Contact lenses, 69, 70 Con vergence, 9 1 C o n verg i n g l e nses, 68, 69 Cooper-Hew itt mercury a rc, 1 9 Cornea, 70, 84, 85 Critical a n g l e , 44, 1 55 C rysta l s , d o u b l e refract i n g , 55 Dark c h a m ber, 79 Darkness, 35 Da y l i g ht, 3 1 , 1 3 1 Depth, 9 1 -93, 94 Desca rtes, 98 Destruct iv e i n terf e renc e, 48-49 Developer, 1 44, 1 46 D i ch r o m a t i s m , 1 1 3, 1 1 5, 1 1 6 D i ff ra ction, 36, 46--4 7 , 50, 77 g r a t i n g s, 58, 62-63 Diffuse reflection, 35, 39, 1 37 D i m i n i s h i n g g l ass, 66 D i opter, 65 D i scord a nt c o l ors, 1 34, 1 35 D i spersion, 45, 62, 63, 66 D i stance, 9 1 , 94, 95 D i sti nctness, 9 1 , 93 D i verg i n g l e nses, 68, 69 Dove p r i s m , 60 Dufayco l o r f i l m, 1 45 Dyes, 1 1 0, 1 40-1 4 1 , 1 44, 1 45, 1 46 Ea rth, 2 1 E d i s o n , T h o m a s, 1 8 · E i n s te i n , energy equation, 7 theory of re l a t i v ity, 7 Ektachrome f i l m , 1 46 E l ectrol u m i nescence, 2 8 E l ectroma g n e t i c w a ves, 8-1 1 , 1 5 1 E l ectron bea m, 77, 1 50 E l ectron m i c roscope, 77 E l evation, 9 1 , 92 E m o t i o n a l effects of col or, 1 33 , 1 36 E m u l s i o n s, 1 44, 1 45 Energy, rad iant, 8, 1 4, 1 5, 27, 29, 98, 1 00, 1 53

1 58

E n l a rgers, 77, 78 Eye, 29, 30, 84-97, 1 09, 1 1 3, 1 1 8 Eyeg l a sses, 68-69, 70 Eyepiece, m i croscope, 77 opera g l a sses, 72 refract i n g te l escope, 73 spy g l a ss, 72 Fahrenheit temperature, 14 Farsi ghted ness, 68 F i be r opti cs, 1 55- 1 56 F i l a m ent l a m p , 1 8, 1 9, 22, 23 F i l m , photogra p h i c , 30, 3 1 , 78, 79, 80, 8 1 , 1 46, 1 47 F i lter, col ored, 1 1 0, 1 1 1 , 1 45 p o l a r o i d , 54 F i x i n g process, photography, 1 44 F l u o rescence, 28, 5 2 F l u o rescent l a m p , 1 9, 23, 25, 26--2 7 f - n u m ber, 80 Foca l l e n g t h , 64; 77, 80 Foca l p o i n t , 59, 64, 65 Focus, 68, 70, 74, 76, 78, 79, 8 1 ' 84, 85 F o ur -co l o r sepa rations, 1 49 Fovea, 86, 87, 89, 1 1 4, 118 F ra u n hofer, J oseph, 62 l i nes, 1 2-1 3, 2 1 Frequency, 8, 1 0, 1 1 F u l l - co l o r rep rod uction, 1 49 Gabor, D e n n i s, 1 54 Ga l i leo, 37, 72 G a m m a rays, 9 Ga n g l i a , eye, 87 Geomet r i c a l opti cs, 58 G l a re, 54 G l a ss f i ber bund l e , 1 55, 1 56 G l o w tu be, 1 9, 24-25, 26 G r a t i n g s , d iffract i o n , 58, 62-63 Gravu re, p r i n t i n g , 1 48 H a l e tel escope, 74 H a r m o n i o u s colors, 1 33 , 1 34, 1 35 Heat ra ys, 1 0 H o l ography, 1 54

H o l og r a m , 1 54 H ue, 1 00, 1 0 1 -1 1 2, 1 1 5, 1 26, 1 27, 1 32, 1 34, 1 35, 1 36 I l l u m i nance, 33, 34 I l l u m i n a t i o n , 29-35 , 76, 77, 78, 93, 99, 1 20, 1 22, 1 4 1 e l ectric, 23 I l l u s i o n s , 90, 96--9 7 I m a g e s h a rpness, 77 I mag e s , 58, 64, 67, 76, 77, 78, 1 08, 1 44, 1 45, 1 55 ; see a lso, rea l i ma g e , v i rt u a l image I n ca ndescent f i l a me n t l a m p, 1 2, 1 8, 1 9, 22-23, 1 3 1 I n c i d e n t f l u x , 33 I n c i d e n t l i g h t, 36, 4 1 , 43, 44, 52, 57 I n fra red r a d i a t i o n , 9, 1 0, 1 5, 27 I n h e r i ta n c e , c o l o r b l i n d ness, 1 1 5 I n ta g l io, 1 48 I n t e n s i ty of l i g h t , 1 5, 30, 32, 33, 82, 99 I n terference, 36, 48-49, 50 I n te r n a l reflect i o n , 44, 60, 76 I nte r n a t i o n a l C o m m i s ­ sion on I l l u m i na­ tion (Com m i ssion l n ternationa l e d e I ' E cl a i ra g e -C I E ) , 1 1 7, 1 30 ; see a lso C I E system I nte r n a t i o n a l sta n d a r d ­ i zed l i g h t sou rces, 17 I r idescent co l o rs, 49 I r is, 84, 85 Ke l v i n temperatu re, 1 4 K od a c h ro m e f i l m , 1 46 L a n d , Ed w i n , 56 camera ( P o l a ro i d ) , 1 47 Lasers, 1 52-1 53, 1 54 Latent i m a ge, 1 44 Lens, opt i ca l , 58, 64-8 1 a berra t i o n s , 6 7 a c h ro m a t i c , 66 ca mera , 79, 80, 8 1 c h ro m a t i c a b e r r a t i o n , 66, 67 c om p o u n d , 7 1

conde n s i n g , 78 contact, 69, 70 converg i n g , 64 correct i n g , 15 c u r ved g l ass, 64 cylind r i ca l , 70 d i ve rg i n g , 66, 69 d o u b l e -concave, 66 d o u b l e -convex, 64 e q u i co n cave, 66 m i c roscope, 7 1 , 77 n e g a t i v e , 66, 69 n e g a t i ve m e n i scus, 66 o b j ecti ve, 7 1 , 73, 76 ocu l a r, 76, 77 p l a n oconcave, 66 p l a n oconvex, 64 pos i t i ve, 64, 68, 69, 73, 77 pos i t i ve m e n i scus, 64 projector, 78 s p h e r i ca l a be r ra t i o n , 67 t e l e p h oto, 7 1 lens, e y e , 6 8 , 69, 70, 84-85, 88, 93 letterpress, 1 48 light absorption, 36, 5 1 , 54, 99, 1 1 0, 1 39 behavior, 36-57 b r i g htness, 32-33, 9 1 , 93, 1 00, 1 03 , 1 20, 1 33 cond u ctors, 1 55 d i ff ra c t i o n , 36, 46-47, 50, 77 d i s pe r s i o n , 45, 62, 63, 66 e n te r i n g the eye, 87 i n c i d e n t , 36, 4 1 , 43, 44, 52, 57 i n t e n s i t y , 1 5, 30, 32, 33, 82, 99 i n te rference, 36, 48-49, 50 measurement, 6, 29, 30, 33 meter, 29 monochromatic, 1 3, 1 0 1 , 1 02, 1 04, 1 07, 1 1 2 , 1 52, 1 53 photons 24, 25, 86 p o l a r i :j:ed , 53-56, 57 rays, 57, 67, 78, 8 1 , 84, 98 reflected, 35, 36, 39-4 1 , 44, 49, 50, 54, 58, 60, 98, 99 refra ct i o n , 36, 40, 42-43, 57, 58, 60

scatte r i n g , 2 1 , 36, 50, 54, 99 see i n g , 84 sou rces, 1 7, 1 8-28, 63 speed of, 7, 37-38, 42, 43 t ra n s m i s s i o n, 36, 1 55 v i s i b l e, 1 0 waves, 6, 8, 9, 1 0, 1 1 w h ite, 1 3, 1 7, 45, 1 00, 1 02, 1 05, 1 1 1 , 1 1 2 l i g h t a n d c o l or, a s too l s , 1 33- 1 56 n a t u re of, 4 - 1 7 l i g htness, 35, 1 03 l i n otype, 1 48 l i ppershey, H a n s , 72 l i t h o g r a p h y , 1 48- 1 49 l u mens, 33 l u m i n a n ce, 32, 33, 1 02, 1 03, 1 2 8, 1 52, 1 53 l u m i n e scence, 28 l u m i nos i ty, 30, 1 1 4 l u m i n o u s f l ux, 33 l u x mea su re, 33 Macu l a l u tea, eye, 86 M a g n i f i c a t i o n , 65, 7 1 , 73, 76, 77 Ma g n i f i e r, pocket d o u b l et, 7 1 s i m p l e, 64 Measu rements, 6, 29, 30, 33 Mercu ry a rc, 1 9, 26 Metric system, 6 M i c h e l so n, A l bert A . , 38 M i croscope, 6 4 , 7 1 , 77 M i rrors, 4 1 , 44, 58-59, 6 1 , 64, 67, 74, 75,

77

Mo noc h ro m a t i c l i ght, 1 3, 1 0 1 , 1 02, 1 04, 1 07, 1 1 2, 1 52, 1 53 M o n o c h rom a t i s m , 1 1 3, 1 14 Monochromator, 63 Monochrome tube, 1 50 Monocu l a r cues, 95 Moo n l i g h t, 86 Mord a n t, 1 40 Mosa i c add i t i ve c o l o r m i x i n g , 1 50 Mosa i c f u s i o n , 1 09, 1 5 1 Mosa i c system f i l m , 1 45 Motion, 91 p a ra l l a x , 94, 95 M u l t i l a y e r f i l m , 1 46, 1 47 M u n se l l , A l be rt, H . , 1 26 c o l o r system, 1 25, 1 26-1 27, 1 30

M u s c l e s , c i l i a r y , 85 N a t i o n a l B u rea u of Sta n d a rd s , 1 27 Nea r s i g hted n e s s , 66, 69 N e g a t i v e l e n se s , 66, 69 Neon, 1 9, 24-25 New to n, S i r I sa a c, 45 Newto n i a n t e l escope, 74 N ic o l p r i s m , 57 N o n - co h e r e n t l i g h t waves, 49, 1 52 Nuclear energy, 1 8 O b ject i v e l e n s , 7 1 , 76 Offset l i t h o g r a p h y , 1 49 Opera g l a s s, 72 O p t i c a x i s, 59, 64 O p t i c c h i a s m a , 88, 89 O p t i c nerve, 85, 86, 87, 88, 89, 90 Optica l i l l u s i o n , 90, 96-97 O p t i ca l i n s t r u m e n t s 58-83 O p t i ca l Society of A m e r i ca , 5 O r g a n i c d y es, 1 4 1 Ostwa l d c o l o r s y stem, 1 25, 1 28-1 29, 1 30 P a i nts, 1 06, 1 1 0, 1 37-1 39 P a r a bo l i c m i rror, 58, 75 P a r a bo l o i d a l m i rror, 74, 75 P a ra l l a x , 94, 9 5 P a sc h e n spectrog raph , 63 P e n u m bra , 34 Percept i o n , 32, 35, 90, 94, 95, 96, 99, 1 1 8- 1 24 P e r i scopes, 60, 1 55 Perspect i ve , 9 1 P h o s ph orescence, 28, 52 P h ospho r s , 52 P h otoe l e c t r i c ce l l , 80 P h ot o g ra p h y , 56, 7 1 , 78, 79-8 1 , 92, 95, 1 44- 1 47 P h otometers, 29, 82 P h ot o m e t r i c u n i t s , 33 P h otometry, 3 2 P h otons, 24, 25, 86, 1 53 P h ot o s p h e re, 2 1 P h oto-typesett i n g , 1 48 P i g m ents, 87, 1 06, 1 1 0, 1 1 8, 1 37- 1 39 P i n h o l e ca m e r a , 79

1 59

P l a n e m i rror, 58 P l a n e of v i b ra t i o n , 54 P o l a color f i l m , 1 47 P o l a r iscope, 55 P o l a r i zat ion processes, 36, 57 P o l a r i zed l i g ht, 53-56, 57 P o l a ro i d , 54, 56, 57 L a n d camera, 1 47 Positive l e n ses, 64, 68-69 Positive transparency, 1 45 P resses, h i g h - speed, 1 48 P r i m a ry c o l o rs, 1 04 P r i n t i n g , 1 48, 1 49 P r i s m s, 55, 58, 60-61 , 62, 63, 76 Projectors, 77, 78 P u p i l , eye, 84, 85 P u rity of c o l or, 1 02, 1 28

Rad i a t ion, 9, 1 0, 1 1 , 1 4, 1 5, 22, 26, 1 53 d e tectors, 29, 90 pyrometry, 2 1 Ra i n bow, 4 , 5 , 98 Rea l i m age, 58, 59, ,65, 66, 77, 1 54 Reflect i n g t e l escopes, 74 Reflect i o n , 35, 36, 39-4 1 , 44, 49, 50, 57, 58, 60, 98, 99 Refl ectors, 6 1 , 7 1 , 74, 78 Refracti n g t e l escopes, 73 Refract i o n , 36, 40, 42-43, 57, 58, 60, 98 Refractive i nd e x , 42, 43, 45, 57, 60 Refractors, 7 1 Regu l a r ref l ecti o n , 3 9 , 1 37 Reso l v i n g , 1 09 Ret i n a , 84, 86, 87, 88, 89, 90, 9 1 , 1 09, 1 1 8 Retinene, 86 R h odops i n , 86 Ridgway, Robert, 1 28 Rods, eye, 86, 1 1 8 Roemer, O l af, 37, 38 Roto g ravu re, 1 48 Row l a nd , H e n ry A., 62

1 60

Sacc h a r i meter, 54 Satura t i o n , 1 00, 1 0 1 , 1 02, 1 03, 1 1 5, 1 26, 1 34, 1 35, 1 36 Scatte r i n g , 2 1 , 36, 50, 57, 99 Sch m i d t camera, 75 Screen, pro jector, 77, 78 Sensation of s i g h t, 90, 98, 1 03 Senses, 4, 1 0, 90, 1 00 Shadowgrams, 9 Shadows, 34, 98, 1 22, 1 33 S i l ver h a l ide crysta l s , 1 44, 1 45 S i m u l taneo us c o l o r contrast, 1 2 1 , 1 24 S i ze, known ( o r i m a g i n ed } , 94 Sod i u m, spect rum of, 25 vapor l a m ps, 25 Sound, speed of, 7 Spectro graph, 5, 1 2, 6 1 , 63 Spectrometer, 6 1 S pectroscopes, 63 Spectru m , 5, 1 0-1 3, 25, 62, 98, 1 1 7 Spherica l a berra t i o n , 67, 75 Sprea d i n g effect, col ors, 1 24 Spy g l ass, 72 Sta n d a rd i l l u m i n a t i o n , 141 Sta n d a rd l i g h t sou rces, 1 7, 83, 1 1 7 Sta n d a rd l u m ino sity curve, 29, 30 , 1 30 Sta rs, c o l o r - te m perature c l a ssification, 1 6 Stereo camera, 95 Su btract ive color m i x ­ i n g , 1 06, 1 1 0-1 1 1 , 1 1 2, 1 39 Successive afte r i m a ges, 1 24 S u n , 1 5, 20-2 1 S u n l a m p, 26 Su n l i g ht, 1 3, 1 7, 2 1 , 98, 99, 1 3 1 Suntan, 9 Su perpos i t i o n , 9 1 , 92 Su rfa ce i l l u m i na t i o n , 34 Suspensory l i gament, eye, 85

Symbo l i s m of c o l o r, 1 3 3, 1 36, 1 43 Te l e p h oto l e ns, 73 Te l e s cope, 64, 72, 75, 76 refract i n g , 73 refl e ct i n g , 74 Te l e v i s i o n , col o r, 1 09, 1 50-1 5 1 Te m p e r a t u re, 1 4 c o l or, 1 4, 1 6 , 1 7 Terrestr i a l t e l e scope, 72 T h e r m a l rays, 1 0 T h e r m o p i l e , 29 Tou r m a l i n e, 53, . 54, 56 Tra n s m i ss i o n of l i g ht, 36 Transpa rency, c o l o r f i l m , 1 45, 1 46 Trichromat, 1 1 3, 1 1 4 Tri p l e m i r ro r, 6 1 T u n g s ten l a m p s, 22, 77, 78 U l t r a v i o l e t rays, 9, 1 5, 26, 27 U m bra, 34 U n ive rsa l constant, 7 U n sa t u rated c o l o rs , 1 36 V i rt u a l i ma g e , 58, 59, 65, 66, 77, 1 54 V i s i o n , 84-97 co l o r, 98- 1 1 7, 1 1 81 24 V i s u a l p u r p l e , 86 V i treous h u m o r, 84 Water wa ves, d i ff raction of, 46 Wave crests, 8, 9, 48 W a v e l e n g t h , def i n i ­ tion, 9 d o m i n a nt, 1 0 1 , 1 1 2, 1 28, 1 30 of i n f r a red ra y s , 1 1 o f l i g h t, 1 0, 39, 46, 77 o f v i s i b l e rays, 1 1 Wel s b a c h g a s m a n t l e , 1 8, 1 9 W h i te l i g ht, see L i g h t X - ra y s , 9 Ye l l ow spot, eye, 86

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