Chapter 5 Light STUDENT'S MODULE

JPN Pahang

Physics Module Form 4 Chapter 5 Light

CHAPTER 5: LIGHT In each of the following sentences, fill in the bracket the appropriate word or words given below. solid, liquid, gas, vacuum, electromagnetic wave, energy 1. Light is a form of ( ). 2. It travels in the form of ( ) 3. In can travel through ( ) 4. It travels fastest in the medium of ( ) 5. Light of different colours travels at the same speed in the medium of (

)

Light allows us to see objects. Light can be reflected or refracted. 5.1 UNDERSTANDING REFLECTION OF LIGHT Plane mirror and reflection: In the boxes provided for the diagram below, write the name of each of the parts shown.

i

r

Plane mirror Laws of Reflection: State the laws of reflection. (i)

……………………………………………………………………………………………. ………………………………………………………………………………………..… ….

i

r Plane mirror

(ii)

………………………………………………………………………………………… …..

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………………………………………………………………………………………… ….. Exercise 1. The diagram below shows how the relationship between incident angle and reflected angle can be investigated. Fill in the values of the angles of reflection, r in the table below

mirror i r

i r

mirror

ON OFF ON

Laser pen

OFF

Laser pen ir1020304050

Exercise 2: Original direction Mirror

θ

d

o

50

normal

Exercise 3:

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Based on the diagram on the left, calculate the angle, θ. Hence determine the angle of deviation, d.

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Physics Module Form 4 Chapter 5 Light

Mirror before rotation

θ Mirror rotated θ o

α norm al

Incident ray

Reflected ray before rotation Reflected ray after rotation

Based on the diagram above, when the mirror is rotated an angle θ, without changing the incident ray, what is the angle rotated ,α, for the reflected ray in terms of θ ?

Image formed by a plane mirror: Using the law of reflection, complete the ray diagram to determine the position of the image.

object

A

B

i1 r1

Eye

What can you say about the line joining object and image? ……………………………………… What can you say about the distances AB and BC? ……………………………………………….. Differences between real and virtual image:

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Real imageVirtual imageCan be caught on a screenCannot be caught on a screenFormed by the meeting of real rays.Form at a position where rays appear to be originating.

Characteristics of image formed by plane mirror: Observe the pictures below as well as using previous knowledge, list the characteristics.

i)

mir

ror

ii) image

object

iii) iv)

Exercise 1: Complete the ray diagram below consisting of 2 rays originating from the object, reflected and entering the eye such that the eye sees the image.

Mirror

object

Eye

Exercise 2:

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Ahmad is moving with speed 2 m s-1 towards a plane mirror. Ahmad and his image will approach each other at 1 m s-1 2 m s-1 3 m s-1 4 m s-1 Exercise 3: Four point objects A, B, C and D are placed in front of a plane mirror MN as shown. Between their images, which can be seen by the eye?

M

N

Eye A

ACTIVITY:

B

C

D

Find out some of the uses of plane mirrors (application of reflection).

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Curved Mirrors: Concave mirror

Convex mirror

C

P r

C

P r

Terminology: Refer to the diagrams above and give the names for the following: C = r = P = PC =

Effect of curved mirrors on incident rays: a) Incident rays parallel to the principal axis:

Concave mirror

C

F

Convex mirror P

P

f

F

f

r

r

Study the diagrams above and fill in the blanks for the following sentences. Rays parallel to the principal axis converge at the ……………………, F F is positioned at the ………………….. between C and P FP is named the ………………………… which is denoted by f. Hence write an equation giving the relationship between r and f.

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C

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Physics Module Form 4 Chapter 5 Light

b) Incident rays parallel to each other but not parallel to the principal axis:

Concave mirror

Convex mirror

Focal plane

Focal plane

C

F

P

C

P F

f

f

r

r

Study the diagrams above and fill in the blanks in the following sentences. 

Parallel rays converge at a point called ……………………………



The focal plane joins F, the principal focus and all …………………………..and is ………………………. to the principal axis



The ray passing through C is reflected back along the line of the…………………….ray.



The distance between the focal plane and the mirror is the ………………………….,f.

Image formed by curved mirror (ray diagram method) Principle of drawing ray diagrams: a. Rays parallel to the principal axis are reflected through the principal focus, F. Example:

C

F

P

P

Convex mirror

Concave mirror

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F

C

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Physics Module Form 4 Chapter 5 Light

Exercise 1: Complete the ray diagrams below:

C

F

P

P

F

C

Convex mirror

Concave mirror

b) Rays passing through the principal focus are reflected parallel to the principal axis. Example:

C

F

P

P

F

C

F

C

F

C

Convex mirror

Concave mirror Exercise 2: Complete the ray diagrams below:

C

F

P

P

Convex mirror

Concave mirror

c) Rays passing through the center of curvature are reflected directly back.

C

F

P

P

Convex mirror

Concave mirror

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Physics Module Form 4 Chapter 5 Light

Exercise 3: Complete the ray diagrams below:

F

C

P

P

F

C

Convex mirror

Concave mirror

Image formed by concave mirror: Using the principles of construction of ray diagram, complete the ray diagrams for each of the cases shown below: u = object distance; v = image distance ; f = focal length ; r = radius of curvature Case 1: u > 2f Concave mirror object

F C

F

Hence state the characteristics of image formed: i)

ii)

iii)

Case 2: u = 2f or u = r Concave mirror object C

F

F

Characteristics of image formed: i)

ii)

iii)

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Physics Module Form 4 Chapter 5 Light

Case 3: f < u < 2f Concave mirror object F

F

C

Characteristics of image formed: i) Case 4:

ii)

iii)

u=f Concave mirror object C

F

F

Characteristics of image formed: i) Case 5:

u 2f

where u = object distance ; and f = focal length of lens. Lens

object

F 2F

F

Characteristics of image:

ii) Case 2 : u = 2f Lens object

F 2F

F

Characteristics of image:

iii) Case 3 : 2f > u > f Lens object 2F

F F

Characteristics of image: iv) Case 4 : u = f

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Physics Module Form 4 Chapter 5 Light

Lens object

F F

2F

Characteristics of image:

v) Case 5 : u < f

Lens object 2F

F

F

Characteristics of image:

Exercise: In each of the following statements below, fill in the space provide one of the following conditions. ( u > 2f / 2f = u / 2f > u > f / u > f / u < f ) i) To obtain a real image, the object must be placed at a distance u such that … ……… ii) To obtain a virtual image, the object must be placed at a distance u such that ………

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Characteristics of image formed by concave lens : (by construction of ray diagrams ) Construct a ray diagram for each of the following and state the characteristics of the image formed i) Lens object

F 2F

F

Characteristics of image: ii) Lens object

F 2F

F

Characteristics of image : Note: Image formed by a concave lens is always diminished, virtual and on the same side of the lens as the object. Power of a lens (p) The power of the lens is given by: Power of lens =

1 focal length

Sign convention (for focal length) and the S.I. unit for power of a lens. • The focal length of a convex lens is (positive/negative) • The focal length of a concave lens is (positive/negative) • The S.I. unit for the power of a lens is…....…and its symbol is… … • When calculating the power of a lens, the unit of the focal length must be in (m/cm) Exercise 1 : A concave lens has a focal length of 10 cm. What is its power?

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Physics Module Form 4 Chapter 5 Light

Exercise 2 : The power of a lens is + 5 D. State whether it is a convex lens or a concave lens and calculate its focal length.

Linear Magnification (m) : Definition:

height of image height of object

Linear magnification =

m=

hi h0

Based of the definition above and the ray diagram below, derive an expression for the relationship between linear magnification, m, the object distance, u and the image distance, v.

B

Lens

ho

O

v C

A D

u

hi

Lens formula : The relationship between the object distance, u, image distance, v, and the focal length, f, of a lens is given by 1 1 1 + = u v f •

This lens formula is valid for both convex and concave lenses.

When using the lens formula, the ‘real is positive sign convention’ must be followed.

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Physics Module Form 4 Chapter 5 Light

The rules stated in this sign convention are: 1) 2) 3)

Application of the lens formula: Exercise 1. An object is placed 10 cm in front of a converging lens of focal length 15 cm. Calculate the image distance and state the characteristics of the image formed.

Exercise 2 :

An object is placed 30 cm in front of a converging lens of focal length 25 cm. a) Find the position of the image, and state whether the image is real or virtual. b) Calculate the linear magnification of the image.

Latihan 3 : An object is placed 30 cm in front of a diverging lens of focal length 20 cm. Calculate the image distance and state whether the image is real or virtual.

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Physics Module Form 4 Chapter 5 Light

Lenses and optical instruments : 1. Magnifying glass (simple microscope ): A lens acts as a magnifying glass when the object is placed as in case 5 on page 23. i) A magnifying glass consists of a (converging / diverging) lens. ii) The object must be placed at a distance (more than f / same as f / less than f / between f and 2f / more than 2f) in order for the lens to act as a magnifying glass. iii) The characteristics of the image formed by a magnifying glass are yang (real / virtual) ; (inverted / upright) ; (magnified /diminished) ; (on the same side as the object / on the opposite side of the object). iv) Greater magnification can be obtained by using a lens which has (long / short) focal length. Complete the ray diagram below to show how a magnifying glass produces an image of the object.

Lens object 2F

F

F

Exercise 1 : A magnifying glass produces an image with linear magnification = 4. If the power of the lens is +10 D, find the object distance and image distance.

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Physics Module Form 4 Chapter 5 Light

Exercise 2: Which of the following lenses with their powers given below makes the magnifying glass with the highest power of magnification? A. – 5 D

B. –25 D

C. +5 D

D. +25 D.

2. Simple camera : The diagram below shows the structure of a simple camera. In the boxes provided, write the names of the parts shown. Focusing screw

Film drum

Diaphragm adjustment ring For each of the parts you have named, state its function.

3. Slide projector : The diagram below shows the structure of a simple camera. In the boxes provided, write the names of the parts shown Screen

Light source

Complete the ray diagram above to explain how the slide projector works.

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Physics Module Form 4 Chapter 5 Light

4. Astronomical telescope : Making of the astronomical telescope. • •

The astronomical telescope consists of 2 (converging / diverging) lenses. The objective lens has focal length, fo and the eye lens has focal length, fe where ( fo < fe / fo > fe ). The lenses are arranged such that the distance between the objective lens and the eye lens is (fo – fe / fo + fe / fo x fe / fo/fe).

•

Parallel rays from distant Objective lens object

Eye lens Fo

Fe

Complete the ray diagram above to show how the astronomical telescope works. Characteristics of image formed by an astronomical telescope: • •

The first image formed by the objective lens is (virtual/real ; upright/inverted ; diminished/magnified). The final image is (virtual/real ; upright/inverted ; diminished/magnified).

•

The final image is located at ( Fo / Fe / infinity).

Magnifying Power (M) : M=

f f

0 e

Exercise: An astronomical telescope with high power of magnification can be built using eye lens of (long / short) focal length and objective lens of (long / short) focal length.

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5. The compound microscope : Structure of the compound microscope: • • • • •

A compound microscope consists of 2 (converging / diverging) lenses The focal length of the eye lens is (long / short) and the focal length of the objective lens is (long / short). The objective lens is arranged such that the object distance, u is (u = fo / fo < u < 2 fo / u =2fo). The eye lens is used as a (magnifying / diverging / projector) lens. The total length, s, between both lenses is ( s = fo + fe ; s > fo+fe )

Le

L0 Object

Fo

Eye

Fe

Complete the ray diagram above to show how the compound microscope works. Characteristics of image formed by compound microscope: • •

The first image formed by the objective lens is (real/virtual ; diminished/magnified ; upright/inverted ). The final image is (real/virtual ; diminished/magnified ; upright/inverted ).

Exercise 1 (a) : A compound microscope consists of two lenses of focal lengths 2 cm and 10 cm. Between them, which is more suitable as the eye lens? Explain your answer.

(b): How would you arrange the lenses in (a) to make an astronomical telescope?

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Physics Module Form 4 Chapter 5 Light

Reinforcement: Part A: 1. Between the following statements about reflection of light, which is not true? A. All light energy incident on a plane mirror is reflected. B. The angle of incidence is always the same as the angle of reflection. C. The incident ray, the reflected ray and the normal to the point of incidence, all lie on the same plane. D. The speed of the reflected ray is the same as the speed of the incident ray. 2. A boy stands in front of a plane mirror. He observes the image of some letterings printed on his shirt. The letterings on his shirt is as shown in Figure 1.

Figure 1 Between the following images, which is the image observed by the boy? D

C

B

A

3. Figure 2 shows an object, O placed in front of a plane mirror. Between the positions A, B, C and D, which is the position of the image? A

B

C

D

Plane mirror O Figure 2 4. A student is moving with a velocity of 2 m s-1 towards a plane mirror. The distance between the student and his image will move towards each other at the rate A. 2 m s-1

B. 3 m s-1

C. 4 m s-1

D. 5 m s-1

E. 6 m s-1

5. The table below shows the characteristics of the images formed by a concave mirror for various positions of the object. All symbols used have the usual meanings. Which of them is not true?

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Position of object u > 2f f < u < 2f u=f u