Physics KISS notes

keep it simple science Key Concepts in Colour Preliminary Physics Topic 1

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Slide 1

Preliminary Physics Topic 1

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The World Communicates

First, Some Revision: ENERGY

TYPES of WAVES

Energy is what causes changes and does “work”. The familiar forms of energy include: • HEAT • ELECTRICITY • KINETIC (energy in a moving object) • POTENTIAL (energy stored, such as the chemical energy in petrol).

SOUND

LIGHT

RADIO SIGNALS

WATER WAVES

Examples of energy which moves around as waves include

MICROWAVES ... and many more

Many forms of energy move around as WAVES. A wave is a carrier of energy. In a wave, energy moves, but matter does not.

ENERGY CONVERSIONS Energy can be converted from one form to another.

This causes the air to vibrate too. Waves of vibration spread out through the air... sound waves.

The strings vibrate.

The air vibrates, but does not go anywhere.

Water waves carry energy across the surface of a pond. The water vibrates up & down, but goes nowhere.

In your mobile phone the SOUND WAVES of your voice are converted to ELECTRICAL signals, then transmitted as RADIO WAVES to your friend, whose phone converts it back again. SOUND

Waves Carry Energy Without the Transfer of Matter KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

X-RAYS

ELECTRICAL

RADIO

In this topic you will learn about waves and their properties and features, and how they they are used for communication. Slide 2

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Wave Types & Properties

The Wave Equation

Speed, Pitch and Loudness

Nature of Sound.

Superposition & Interference

1. Waves

Graphing Waves The EM Spectrum

2. Sound Waves

The World Communicates

3. Electromagnetic Waves 4. Reflection & Refraction

5. Digital Communication & Storage KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Light & Mirrors. Reflection in Communication. Slide 3

Inverse Square Law Production, Detection, Dangers EM Waves in Communication

Law of Reflection

Refraction & Snell’s Law. Light, Lenses & Total Internal Reflection Usage & copying is permitted according to the Site Licence Conditions only

1. THE NATURE OF WAVES

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Waves Carry Energy Waves carry energy, without the transfer of matter. This can occur in 1 dimension:

Describing Waves

Pulses moving along a slinky spring

A wave is a vibration. In a mechanical wave, the “particles” (atoms & molecules) in the medium vibrate to transmit the wave energy. In EM waves the vibration occurs in electric and magnetic fields.

Compressed sections in the spring move along it like a “Mexican Wave”... energy is transferred, but the coils merely oscillate back and forth and do not actually go anywhere.

Consider a wave in a rope which has been given a single up-and-down “twitch”: part of the rope (medium) CREST A PULSE WAVE vibrates up & down

Energy moves along the rope

... or in 2 dimensions: Ripples spreading on the surface of a pond.

rope TROUGH

Energy moves along the rope, but the rope itself doesn’t go anywhere. Particles of the “medium” (the rope fibres) vibrate up-and-down as the energy moves across. If the rope is wiggled constantly up-and-down, you get not just one pulse, but a periodic wave with one pulse following another.

...or in 3 dimensions,

A PERIODIC WAVE

such as when light radiates in all directions from a glowing object.

Energy moves

Waves & Mediums Mechanical waves are those which need a “medium” to travel through. For example, a water wave must have water to travel in. Sound waves need air, or water, or some substance to move in. They CANNOT travel in a vacuum. Electromagnetic (EM) waves do NOT need a medium... they can travel through a vacuum, and in fact travel fastest in a vacuum. EM waves include light, radio waves, ultra-violet and other types, and are studied in detail in a later section. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 4

CREST

TROUGH

MECHANICAL WAVES require a medium to travel through. ELECTROMAGNETIC WAVES do not. A PULSE WAVE is a single wave disturbance. PERIODIC WAVES contain a series of pulses, with a continuous set of crests and troughs. Usage & copying is permitted according to the Site Licence Conditions only

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Transverse & Longitudinal Waves Longitudinal waves are when the particles of the medium vibrate back-and-forth in the same line as the energy moves. For example, when a series of “compressions” and “rarefactions” are sent along a slinky spring.

A PERIODIC, TRANSVERSE WAVE Energy moves

Rope vibrates up and down

CREST

TROUGH

LONGITUDINAL WAVE IN A SPRING Energy moves

This form of a wave, where the medium vibrates at right angles to the direction that the energy moves, is called a Transverse Wave.

compression in spring

Spring vibrates

rarefaction (where spring is stretched)

TRANSVERSE WAVES vibrate at right angles to the direction that the energy is moving.

LONGITUDINAL WAVES vibrate back-and-forth in the same direction that the energy is moving.

Energy flow Vibration in medium

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Earthquake Shock Waves occur in different forms, both Transverse & Longitudinal. Slide 5

Energy flow Vibration in medium Usage & copying is permitted according to the Site Licence Conditions only

Wave Measurements

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All periodic waves, whether Longitudinal or Transverse, Mechanical or Electromagnetic, can be described and measured by their:-

Wavelength = the distance from one crest to the next. (or from one trough to the next, or from one compression to the next) The S.I. unit is the metre (m).

Period

The Greek letter “lambda” wavelength.

Note that there is a simple relationship between Frequency and Period... they are reciprocals.

λ is used as the symbol for

Amplitude (a or A) = the distance that a particle in the medium is displaced from its “rest position” at a crest or trough. i.e. the maximum displacement distance.

Frequency (f) = the rate at which the wave is vibrating. Frequency is the number of waves that pass a given point in 1 second, or the number of complete vibrations per second. S.I. unit is the “hertz” (Hz)

(T) = the time (in seconds) for one complete vibration to occur.

T= 1 f

and f = 1 T

Velocity (v) = the speed of the wave, in metres/sec.(ms-1) There is a simple relationship Wavelength and Frequency:

between

Velocity,

1 Hz = 1 wave per second.

THE WAVE EQUATION

WAVELENGTH

Velocity = Frequency x Wavelength Wave cycles per second is FREQUENCY KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

V = fλ

AMPLITUDE

Slide 6

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Wave Equation Calculations

Example Problem 1 A water wave in the ocean has a wavelength of 85m, and a velocity of 4.5ms-1. a) Find the frequency. b) What is the period?

Solution

V= fλ 4.5 = f x 85 f = 4.5 / 85 = 0.053 Hz (5.3 x 10-2 Hz) (i.e. only a small fraction of a wave passes by each second.)

Example Problem 2 A sound wave has a period of 2.00x10-3s. (T= 0.002s) Sound travels in air at a velocity of 330ms-1. a) What is the frequency of the wave? b) Find the wavelength.

a)

b)

T=1/f = 1 / 0.053 = 19 s (i.e. it takes 19 seconds for 1 complete wave, crest to crest, to pass by) KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 7

Solution a)

f =1/T = 1 / 0.002 = 500Hz (i.e. 500 vibrations per sec.)

b)

V=fλ 330 = 500 x λ λ = 330 / 500 = 0.66m (i.e. 66cm from crest to crest)

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Graphing Waves

A good way to represent a wave is by using a graph.

Imagine a floating cork bobbing up and down as a series of ripples move across the water surface (i.e. a periodic wave). Ripples Cork bobs up and down

+3 0

One period = 0.8 s 0.2

0.4

0.6

Time (s) 0.8

1.0

1.2

-3 3

Displacement (cm)

If you graph the (up-down) displacement of the cork against time, the graph will look something like this:

Be careful! The graph is shaped like a wave, so it’s tempting to try to read the wavelength from the horizontal scale... but the horizontal scale is TIME, not length.

What you CAN read from a Displacement-Time graph:

Amplitude The vertical scale measures the displacement of the cork from the “equilibrium” position (i.e. the flat water surface). So, at 0 sec, the cork was in the equilibrium position. at 0.2 sec, it was 3cm upwards... at 0.4 sec, it was back at equilibrium... and so on. Its maximum displacement was 3cm either above or below (d= -3cm) equilibrium, so the Amplitude = 3cm (0.03m) Period

Since the horizontal scale is time, you can easily read from the graph how long it takes for one complete up-and-down cycle. On this graph T = 0.8s From Period, calculate Frequency:

f=1/T = 1 / 0.8 = 1.25Hz If the speed of the wave was known, then you could calculate the wavelength, or vice versa. e.g. if the ripples are 0.45m apart: (i.e. λ = 0.45m) V=fxλ = 1.25 x 0.45 So, velocity = 0.56 ms-1

Graphing a Longitudinal Wave You might think these Displacement-Time graphs wouldn’t work for a Longitudinal wave where the particles vibrate back-and-forth rather than up-and-down. However, the graph of a longitudinal wave can be exactly the same... you just have to realise that the “displacement” is sideways displacement from the “equilibrium position”, instead of up-down displacement. Amplitude, Period and Frequency can all be determined in exactly the same way.

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Relationship Between Wavelength & Frequency

You may have carried out a “First Hand Investigation” in class to see how a change in Frequency (at constant velocity) affects the wavelength. Maybe you used a slinky spring, or watched the water waves in a “ripple tank”.

Longer Wavelength

Lower Frequency

You would have found... INCREASING the FREQUENCY

DECREASE in WAVELENGTH

Shorter Wavelength

Higher Frequency

and DECREASING INCREASE in the FREQUENCY WAVELENGTH (If VELOCITY is the same) KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 9

To have the same speed, the shorter waves must vibrate at a higher frequency Usage & copying is permitted according to the Site Licence Conditions only

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Activity 1

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

WAVES

Student Name .................................

1. What is the difference between: a) a mechanical wave and an electromagnetic (EM) wave? b) a pulse wave and a periodic wave? c) a transverse wave and a longitudinal wave? 2. These 2 waves are sketched to the same scale and they travel at the same speed. Which wave (P or Q) has the: a) longer wavelength? b) larger amplitude? c) higher frequency? d) longer period? KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Wave P

Wave Q

Slide 10

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2. THE PROPERTIES OF SOUND WAVES

Sound Waves

Velocity of Sound

Sound waves are Mechanical (they need a medium) and Longitudinal (vibrate back-and-forth in the line of the energy flow) SOUND WAVES

Sound travels at different speeds in different mediums. In air, sound travels at about 330-350ms-1, (about 1,200 km/hr) depending on temperature and density.

Energy moves Particles vibrate

The denser the air, the slower the speed of sound.

Instead of crests and troughs, a series of “compressions” and “rarefactions” pass through the medium as a sound travels. The atoms and molecules are alternately “squashed together” and then stretched apart as the energy flows through. Sound Travels

Compression

FREQUENCY = “PITCH”

Rarefaction

Compression

When you hear sounds of different “pitch” that is the way your brain interprets sound waves of different frequency. Rarefaction

Low Frequency = Low Pitch

In a compression the air pressure is higher, and lower in a rarefaction. Displacement from the equilibrium

Compressions.

The back-and-forth vibration of the medium produces a typical wave shape if graphed.

In liquids and solids, sound travels much faster... ...about 1,500ms-1 in water ...about 5,000ms-1 in most metals.

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Higher air pressure

AMPLITUDE = LOUDNESS or VOLUME Sound waves with different amplitudes are interpreted by your brain as sounds of different loudness or volume.

Time

Rarefaction.

High Frequency = High Pitch

Larger Amplitude = Louder Sound Smaller Amplitude = Quieter Sound

Lower pressure

Slide 11

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ECHOES

...ECHOES

...ECHOES

Like all waves, sound can travel through a medium like air, strike another medium (say, a brick wall) and bounce back. The REFLECTED wave will be heard as an echo.

BAT

USES OF SONAR

The time delay between sending a sound “ping’ and receiving the echo, gives depth and distance

SONAR SOund Navigation And Ranging Anti-S Submarine Warfare

Echoes from i nsect

Depth Sounding

“Squeaks” of sound

Some animals can send out sound waves and pick up the echoes to help locate their prey, or to navigate, in environments where they can’t see very well, such as murky water (dolphin), or in darkness (bat). KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 12

Humans have invented SONAR technologies for things such as “depth sounding” and detecting underwater objects... fish or submarines, it all works the same way. Usage & copying is permitted according to the Site Licence Conditions only

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The Principle of Superposition

All waves have the ability to pass through other waves without being affected. For example, you could shine a red spotlight across a beam of blue light, each colour and beam will emerge on the other side exactly the same.

However, if the waves are “out of phase” (for example, if compression coincided with rarefaction) then there is destructive interference... the opposite amplitudes may cancel each other out.

However, for the instant that the 2 waves are superimposed upon each other, they do interact and “interfer” with each other.

Add positive & negative displacements at the circled points Displacement

Displacement

“resultant” A+B wave A wave B To find a “resultant”, add the displacements of A&B at convenient points (circled)

Very simply, the displacement of the two waves add together at every point where the waves coincide.

In this case, the waves A&B were “in phase” (crest co-incided with crest, trough with trough) so the result was constructive interference... the resultant has an amplitude which is the sum of A+B. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 13

wave A Resultant wave B

Theoretically, if 2 sound waves had the same amplitude and were perfectly “out of phase” they could cancel out totally... imagine having 2 sounds that add up to SILENCE! (or 2 lights that combine to form DARKNESS!) In practice, this only happens over short distances or time periods to give “interference patterns” and “beat sounds”. Usage & copying is permitted according to the Site Licence Conditions only

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Activity 2

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

SOUND WAVES

Student Name .................................

1. Describe a sound wave using 2 technical words. 2. a) If how b) If how

you listened to sound waves of different frequency, would they sound different? you listened to sound waves of different amplitude, would they sound different?

3. What does “SONAR” stand for? Outline how it works

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wave C

wave A

Displacement

Displacement

4. For each pair of graphed waves, use the Principle of Superposition to sketch the graph of the “resultant” wave. Describe the type of interference in each case. wave B

Slide 14

wave D

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3. ELECTROMAGNETIC WAVES

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EM Waves

Electromagnetic waves are Transverse waves which do NOT require a medium to travel through. They travel through a vacuum at 3.00x108ms-1, the “speed of light”. They can travel through many other substances at slightly slower speed. For example, light can travel through glass or water at speeds of around 2.5x108ms-1. In air, the speed is so close to the speed in a vacuum that, for simplicity, (K.I.S.S. Principle) we take it to be the same. EM radiation does not require a medium because the waves propagate as vibrations of electric and magnetic fields, not as vibrating particles.

low

v.long

MEMBERS OF THE EM SPECTRUM Radio (and TV) waves

visible LIGHT

very short

ultra-violet X-rays Gamma rays

For example.... Radio waves are produced by electric currents running back-and-forth in a conducting wire. Infra-red waves are made by molecules vibrating rapidly because of the heat energy they contain. Light is emitted when electrons rapidly “jump” down from a higher to a lower orbit around an atom.

Frequency increasing

infra-red (heat radiation)

All EM waves are produced in basically the same way: vibration or oscillation of electrically charged particles.

Gamma waves come from the vibrations of charged particles within an atomic nucleus, during a nuclear reaction in the atom. very high

Wavelength decreasing

microwaves

Production of EM Waves

Although we tend to think of these as 7 different types of radiation, you must realise that they are really all the same thing, just at different wavelengths and frequencies. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 15

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Detection & Reception of EM Waves

Just as all EM waves are produced in the same basic way, they are all received or detected in the same basic way too... by a phenomenon called “Resonance”. When waves strike something and are absorbed, they may cause “sympathetic” vibrations within it. In cartoons and the movies (not in real life) the opera singer hits a high note and all the wine glasses begin to vibrate and then shatter... a fictional example of resonance. Some real examples... Antenna

In a film camera the light causes resonance in chemicals in the film. Chemical reactions occur which permanently alter the film so that an image appears when “developed” later.

When the fat lady sings...

When radio waves hit a suitable aerial wire or antenna, they cause some electrons in the metal to oscillate backand-forth “in sympathy” with the wave. These oscillations are amplified electronically and the signal converted to sound in the speaker, allowing you to listen to the radio.

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When infra-red waves hit your skin they cause certain molecules to begin to resonate and vibrate. This sets off nerve messages to the brain and you feel warmth or heat on your skin.

Slide 16

Different film can be sensitive to infra-red, (photos in the dark) or X-rays for medical uses. All waves are detected when they cause resonance vibrations. Usage & copying is permitted according to the Site Licence Conditions only

Danger of High Frequency EM Waves

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High frequency EM waves (ultra-violet, X-ray & gamma) can be very dangerous to living things.

A little UV gives you a suntan, but long-term exposure leads to skin damage, premature skin “ageing”, and is a major cause of deadly melanoma skin cancer. The Sun produces dangerous quantities of UV radiation, but luckily most of it is absorbed by the “ozone layer” in the upper atmosphere of the Earth.

Sun

X-rray & gamma

UV rad io

some reflected

e on inf oz rar ed &l igh t

upper atmosphere KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

“Ozone” is a form of oxygen which has 3 atoms per molecule (O3) instead of the normal 2 (O2). The ozone molecules resonate well at the frequency of UV and so absorb it strongly.

Oxygen O2 does not Absorb UV

UV Rays

The Sun only produces small Ozone O amounts of the even more dangerous Absorbs3 X-rays and gamma radiation. Once UV again, most is absorbed in the upper atmosphere, this time by ordinary oxygen and nitrogen gases.

r ye la

Earth’s surface

Infra-red and light radiation penetrate well, (although about 30% is reflected) and while some radio frequencies get through, many get absorbed or reflected. Slide 17

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The Inverse Square Law

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As any form of radiation spreads out from its source its intensity gets less. For example, a sound becomes quieter if you’re further from the source, or a light is not so bright as you move further from it.

Mathematically, the relationship is that the intensity (I) (such as brightness of light) is inversely proportional to the SQUARE of the distance (d²) from which it is viewed. Intensity α I α

1 (distance)2

α” means “α “proportional to”

1 d2

This diagram explains why:

d” e “2 anc t s i d x light source

dista nce “d”

Square Area

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x2

Square with sides twice as long. 2x

At distance “d” from the light source, some light energy falls on an area of x2 units. At twice that distance (2d) the same amount of light would fall on an area of 4x2. The brightness of the light must be only 1/4 as much (since the same amount of light is falling on 4 times the area.) So, twice the distance

1/4 as bright

3 times the distance

1/9 as bright

10 times the distance

1/100 as bright

...or if you move closer it will getter brighter: at half the distance, 4 times brighter. at 1/3 the distance, 9 times brighter ...and so on.

Area = 4x2 Same amount of light falls on 4 times the area

Notice how the brightness (intensity) changes in proportion to the distance squared, in each case. Slide 18

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Activity 3

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

ELECTROMAGNETIC WAVES

Student Name .................................

1. List, in order of increasing frequency, the main types of EM wave. 2. a) Outline the general way that all EM waves are produced. b) Outline the general way that all waves are received or detected. 3. a) Which 3 forms of EM radiation are dangerous to living things? b) What is ozone? Where is the “ozone layer”? Why is it important to life on Earth?

4. When measured from a distance of 2 metres, the intensity of a light bulb was found to be 16 units. What intensity would be measured at a distance of: a) 4 metres? b) 8 metres? c) 1 metre? KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 19

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EM Waves & Communication

Humans rely on sound waves for communicating by direct speech, but all our modern communication technologies rely on EM waves.

Radio & Microwaves carry radio

Light

and TV broadcasts, telephone longdistance links, mobile phone networks, and satellite links for telephone (including internet) and TV.

is being increasingly used in the form of LASER beams carried in optical fibres for telephone and internet communication.

What’s special about LASER LIGHT?

If you have “Satellite TV”, the “dish” on your roof is an antenna to receive microwaves directly from an orbiting satellite.

•It is one, pure frequency of light. •The waves are all in phase and so they interfere constructively to form a very intense, tight beam.

Add to that, 2-way radio for military uses, CB amateurs and boating, shipping and aircraft communications, and you begin to realise how many radio waves are zapping around. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

•A laser beam will stay inside an optical fibre and not “leak” out or dissipate for long distances. Slide 20

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•A laser can be turned on & off very rapidly, so it’s perfect for high speed digital communication.

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How a Wave Carries Information

How can a voice or piece of music be carried by a wave? The key feature is “Modulation” of the wave. There are 3 common ways to modulate the wave to carry information...

Frequency Modulation (FM) Amplitude Modulation (AM) The frequency (and wavelength) of the wave stays constant while the amplitude varies. The changing amplitude “codes” for the information being carried... whether voice or music, or whatever.

WAVE MODULATION

The amplitude stays constant while the frequency (and wavelength) vary within a fixed range. The information (voice, music etc) is “coded” in the variations of frequency. FM radio gives much better fidelity and is superior, compared to AM, for the quality of sound (eg for music) received. This diagram compares the effect of

AM, FM & Digital Modulation on the same “carrier wave”

Pulse Modulation

“Carrier wave”

No information carried

AM signal

Amplitude changes. Frequency constant

(Digital) Optical Fibres carry Pulse Modulated laser beams KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

To carry information in digital form the wave must switch rapidly between 2 different states, representing the “1” and “0” of digital codes. The wave can be switched rapidly on and off (as in the diagram) or switched back-and-forth between different “phase states”... phase modulation. Slide 21

FM signal

Freq. changes. Amp.constant

Wave pulses on and off

Digital signal Digital data

1

0

1

1

0

1

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Case Study: MOBILE (CELL) PHONES When you use a mobile phone, the sound of your voice goes into a microphone and instantly pops out the other end into your friend’s ear. What happens in between?

2. The digital RADIO signal is transmitted by your phone and received by the local “cell” antenna.

1. The SOUND energy of your voice is converted to ELECTRICAL signals by the microphone. The electrical signal is used to digitally modulate a RADIO wave.

3. If your call is going to a person in another location (a different “cell”) the signal is converted into a modulated MICROWAVE and beamed, via hilltop relay towers, to the correct area. (Alternatively, it might be sent as a modulated Laser LIGHT beam through optical fibres).

4. In the other cell area, the signal is converted back to a modulated RADIO signal and transmitted.

5. Your friend’s phone receives the RADIO signal, amplifies it as an ELECTRICAL signal and this is converted to SOUND waves in their earphone.

ENERGY CHANGES SOUND

ELECTRICITY

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RADIO Slide 22

MICROWAVE (or LASER LIGHT)

RADIO

SOUND

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Discussion: LIMITATIONS OF COMMUNICATION CHANNELS

Modern communication systems have developed rapidly and new features and capabilities seem to come out every day. It seems that the entire system is unlimited and that it can continue to expand and improve forever. Well perhaps it can, but NOT while continuing to use the radio end of the EMR spectrum. Each “station” or channel must operate on a different frequency or else signals can “jam” or “interfere” with each other. The simple fact is that there are now so many radio & TV stations, mobile phone networks, aircraft and shipping channels, military, police and emergency service channels, etc. etc. all using the RF (Radio Frequency) part of the EMR spectrum, that it is becoming difficult to keep expanding services without interfering with existing channels. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 23

In the future we will need to switch more communications to use the laser light & optical fibre method wherever possible, and to make better use of the RF bands. For example, it is possible to use the same frequency “channel” for several different purposes as long as the different signals are modulated differently and as long as the radio receivers are sophisticated enough to pick out only the desired signal and ignore the others. One thing is for sure... humans will keep communicating and the need for new services will keep expanding. So far, our technology has always managed to keep up, and it will probably continue to do so. Usage & copying is permitted according to the Site Licence Conditions only

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Activity 4

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

EM WAVES & COMMUNICATION

Student Name ........................

1. To carry information, waves need to be “modulated” in some way. Name the 3 common ways that signals are modulated and outline each.

2. Use a simple energy change diagram to describe the energy changes occurring during a mobile phone call. 3. Generally, the higher the frequency of a wave the more data and information can be carried. a) Relate this fact to the increasing use of microwaves and lasers in communication. b) The lasers used in communication consist of light rays which we perceive as “red” in colour. Communication engineers are very keen to develop blue-light lasers. Why?

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Slide 24

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4. REFLECTION & REFRACTION

When a Wave Hits a Boundary When a wave is travelling through one medium and then strikes a different medium, one of 3 things can happen at the boundary: Example: Light waves travelling in air, then hitting glass. ABSORPTION of the energy

REFLECTION (bounces off)

Reflection The “Law of Reflection” is very simple: Whatever angle a “ray” of light hits the surface, it will bounce off again at the same angle. OR, more technically: Angle of = Angle of Incidence Reflection

Absorbed energy becomes heat

In cid en tr ay

“Normal” line

io = ro

TRANSMISSION into the new medium, with possible REFRACTION (change of direction)

io

ro

ed ct e l f Re ray

Reflective surface such as a mirror

The trickiest bit is how the angles are measured. They must be measured between the rays and the “NORMAL”... an imaginary line at right angles to the surface.

What if the Surface Isn’t Flat? It is quite possible that all 3 things can happen at once. For example, if a beam of light is travelling through air, and then strikes a glass window: • the glass ABSORBS some of the light. • some REFLECTS off the glass • some is TRANSMITTED through the glass. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 25

The Law of Reflection is still obeyed, as shown: The Incident rays P,Q & R are parallel. Each obeys the Law of Reflection, but the reflected rays go in different directions.

P

Q

R

The “Normal” for each ray is a dotted line.

Uneven, rough surfaces don’t give “shiny” reflections because the light is scattered in all directions. Usage & copying is permitted according to the Site Licence Conditions only

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Reflection of Light from Curved Mirrors

“Concave” mirrors (“go in like a CAVE”) reflect light to a “Focus”, or “focal point”.

Focus

“Virtual” Focus

Concave mirrors can give ENLARGED images if viewed from the right distance, such as a household shaving mirror or make-up mirror, which gives a magnified reflection of your face. This is also the basis of a reflecting telescopes which is the main type used in Astronomy. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

“Convex” mirrors reflect light so the rays diverge outwards, as if coming from a focus behind the mirror.

Slide 26

Convex mirrors produce smaller (“diminished”) images, but give a wider-angle view. An example of use is the side mirrors on a car which give you a wide-angle view into the driver’s “blindspot”. (BUT things look smaller. This can confuse a driver into thinking that other cars are further away.) Usage & copying is permitted according to the Site Licence Conditions only

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Reflections in Communications

Wave reflection from the ionosphere can help with long distance radio communications. It works best with the longer wavelength AM signals. Ion os lay pher er e

Another example involves how Microwaves are transmitted and received. Microwaves are used to relay TV programs to regional transmitters and to relay long distance phone calls (including internet) from city to city. At the transmission end, a curved reflector keeps the waves in a tight beam aimed at the next relay station. The receiver has a similar dish to focus the waves into the receiving antenna.

EARTH Receiver Transmitter

The Ionosphere is a zone in the upper atmosphere where the air molecules are partly ionised (electrically charged) by radiations from the Sun. The ionised gases act as a reflective surface to radio waves of certain wavelengths. TV signals and FM (shorter wavelengths) radio do not reflect so well and generally you need to be in “line of sight” from the transmitter to get good reception. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 27

Microwave Reflector Dishes Microwave beam travels between relay stations Transmitter dish

Your satellite TV dish is a reflector too

Receiver dish

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Refraction

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Refraction occurs when waves enter a new medium. The waves change their speed and their wavelength and, depending on the angle of incidence, may change direction. All waves can undergo refraction, but here we will concentrate entirely on light waves.

When a light wave enters a more dense medium: (Example: going from air into glass)

When going from a more dense, to a less dense medium the opposite changes occur.

• The velocity slows. • The wavelength gets shorter. • The beam changes direction towards the normal.

• The velocity increases: wave speeds up • The wavelength gets longer. • Wave refracts away from the normal.

Incident Ray

Angle of Incidence

io

normal

io > ro Angle of Refraction

Refracted Ray

io

ro

Refracted Ray Air

Glass

Incident Ray

In this case,

Glass

ro

normal

Air

io < ro

When a light ray refracts, its wavelength changes, but frequency stays the same. Since COLOUR is determined by frequency, there is no colour change during refraction. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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You may have carried out an investigation in class using a “Ray Box Kit” to measure angles of incidence and angles of refraction of light rays passing into a glass block.

Angle of incidence, io

When you graph the angles the result is a curve.

In 1621, Willebrord Snell discovered that if you graph the Sine ratios of the angles, the points lie in a straight line. You may have done the same with your experimental data. The fact that it’s a straight line means there is a direct relationship between Sin i and Sin r.

Sin io

Snell’s Law

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i Sin r = Sin e ris n = u r t en di a Gr

The gradient of the Sin ro line is not only the ratio between the Sine of the angles, but is also equal to the ratio of velocities of the wave in the 2 mediums involved. Angle of refraction,

ro

This special ratio is known as the “REFRACTIVE INDEX” (n)

This is not much use for defining any relationship that may exist. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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This is now called Snell’s Law: Sine (angle incidence) = velocity (medium 1) = n Sine (angle refraction) velocity(medium 2) Sin i = V1 = Sin r V2

1n2

Refractive Index

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When waves enter a new medium, and then exit it again, the refractions that occur on the way in, are the opposite of what happens on the way out.

3 beams of light being refracted through a perspex block.

For example, this light ray goes from air, into glass and out into air again. glass 45o

28o

Refraction air -> > glass

Refractive Index (air -> glass)

normal

28o

Refraction glass -> > air 45o

ng = sin45 / sin28 = 1.5

a

and

Refractive Index (glass -> air)

The spoon appears “broken” at the surface of the tea due to refraction of the light by which we see it.

na = sin28 / sin45 = 0.66

g

These 2 values are RECIPROCALS !! ...and this will always be the case... the index of refraction going in is the reciprocal of the index coming out. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Slide 30

1

n2

=1 2

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Snell’s Law

Snell’s Law Calculations Sin i = V1 = Sin r V2

1n2

Example Problem

A beam of light goes from air into a glass block with a refractive index of 1.50. The angle of incidence is 35o. a) Find the angle of refraction. b) If light travels in air at 3.00x108 ms-1, find the velocity in the glass.

Solution a) Sin i = n Sin r

sin 35 / sin r = 1.50

therefore, angle of refraction, b)

V1 = n V2

sin r = sin 35 /1.50 = 0.38238 r = 22.5o

3.00x108 / V2 = 1.50

V2 = 3.00x108 / 1.50 therefore, velocity in glass, V = 2.00x108 ms-1

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Total Internal Reflection & the Critical Angle Consider the situation when waves are going from ®

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a more dense medium into a less dense medium, such as light going from glass into air. The waves refract away from the normal. 1

air glass

io

ro

2

Now think about increasing the incident angle as shown in this series of diagrams.

There comes an angle of incidence (called the “Critical Angle”) where the angle of refraction = 90o. At this point the refracted ray runs along the edge of the glass, but does not cross the boundary.

So, when the angle of incidence equals the “critical angle”, the angle of refraction is a right angle. If io = co, then ro= 90o Remember that

bigger i, bigger r io

ro

Sin i = Sin r

so at the critical angle

3

Sin c = Sin 90

and sin 90o = 1, so... io=co

Critical Angle KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

ro= 90o

gna

gna

Sin c = 1

gna

= 1 ang

This means that the Sine Ratio of the critical angle “C” is equal to the reciprocal of the refractive index of the glass. Slide 32

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What Happens Beyond the Critical Angle?

At incident angles larger than “c”, the ray reflects back inside the glass... this is called “TOTAL INTERNAL REFLECTION” 4

This diagram follows on from the previous slide.

Ray reflects inside glass

io>co

If io > co the ray cannot get out. It reflects back inside the glass.

This has one very important application in communication technology... Optical fibres are thin strands of very pure glass that can carry communications signals in the form of laser light beams. The laser beams stay within the fibres because of total internal reflection.

Each fibre is a core strand of glass, with another layer wrapped around it. The outer layer has a lower refractive index than the core, so even where the fibre bends around a corner, the laser light will generally strike the boundary at an incident angle greater than the critical angle. Whenever the laser beam hits the boundary between the 2 layers, the angle of incidence exceeds the critical angle, (io > co) so Total Internal Reflection occurs and the beam stays totally within the fibres over long distances. Optical fibre laser b eam

Core. High index. Lower index outer layer.

The laser light “bounces” around corners by total internal reflection

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Activity 5

The following activity might be completed by class discussion, or your teacher may have paper copies for you to do.

REFLECTION & REFRACTION

Student Name .................................

1. List the 3 possible things that can happen when a wave strikes a boundary between 2 different media, such as when light strikes a piece of glass. 2. a) Which shape of curved mirror can give enlarged images? b) Outline a use for the opposite shape of mirror. 3. List the changes which can occur to a light wave as it travels from air into a denser medium such as water or glass. 4. (fill in the blank spaces) “Refractive index” can be measured as the ratio between ..................................... of the angles of ............................. and .........................., OR as the ratio between the .............................. of light in each medium. The index for light entering a medium is the ............................... of the index for light exiting from the medium. 5. a) What is the “critical angle” for refraction as light exits from a denser medium? b) Under what conditions does “total internal reflection” occur? KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

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5. DIGITAL COMMUNICATION & DATA STORAGE Digital Technology

In the past 20-30 years our society has become more and more “digitised”. Because of the speed, storage capacity and processing ability of computers, almost every aspect of our society has “gone digital”. A simple list of some of the technologies involved is: This simply means that all information (data) whether it be a person’s voice, written words, numbers, music, photos, etc. is converted into digital code for processing, storage or transmission and communication.

CD’s & DVD’s, Mobile phones, Digital cameras, Computers & Internet, MP3 music, ATM’s GPS Increasingly, WAVES are involved in these technologies, especially when data is moved around... COMMUNICATION.

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TECHNOLOGY CASE STUDY:

GLOBAL POSITIONING SYSTEM

GPS is a system that allows a ship, aircraft, car or a bushwalker, to locate their exact position anywhere on Earth instantly and continuously. The system was developed for miltary uses, but then made available to anyone. The military version is thought to be accurate to within a metre, the civilian version to within about 10 m. The system is based on a fleet of 32 satellites (controlled by the US Air Force) positioned in orbit so that from anywhere on GPS Earth, at any moment, several satellites are in “line of sight”. Each satellite constantly sends out microwave signals identifying itself, its orbit details and the precise time the signal was sent. KCiC Physics 1 World Communicates copyright © 2009 keep it simple science www.keepitsimplescience.com.au

Satellite orbits

Satellite 1

When your portable GPS receiver picks up the signal, it can calculate your exact distance from the satellite, from the time delay since the signal was sent.

(GPS)

GPS receiver

Earth Satellite 3

Satellite 2

By doing the same for 2 other satellites, the GPS unit rapidly “triangulates” the signals from 3 satellites to pinpoint your location on the Earth’s surface. (Aircraft need a 4th signal to get their altitude) GPS systems for cars show your position on a screen overlaid onto a road map of the area. As you drive around, the system constantly shows your changing position, and can advise you where to turn to reach your destination.

Slide 36

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