Physics KISS notes

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keep it simple science

Key Concepts in Colour

Preliminary Physics Topic 1

The World Communicates

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KCiC Physics 1 World Communicates ®

keep it simple science

Preliminary Physics Topic 1

The World Communicates

First, Some Revision:

ENERGY

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). Many forms of energy move around as WAVES.

A wave is a carrier of energy.

In a wave, energy moves, but matter does not.

The strings vibrate.

This causes the air to vibrate too.

Waves of vibration spread out through the air... sound waves.

The air vibrates, but does not go anywhere.

Waves Carry Energy

Without the Transfer of Matter

Water waves carry energy across the surface of a pond. The water vibrates

up & down, but goes nowhere.

TYPES of WAVES

Examples of energy which moves around as waves include

SOUND LIGHT

RADIO SIGNALS WATER WAVES

MICROWAVES

... and many more

ENERGY CONVERSIONS

Energy can be converted from one form to another. In your mobile phone the SOUND WAVESof your voice are converted to

ELECTRICALsignals, then transmitted as RADIO WAVES to your friend, whose

phone converts it back again.

SOUND ELECTRICAL RADIO

In this topic you will learn about waves and their properties and features, and how they they are used

for communication.

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Wave Types &

Properties Nature of_Sound.

Speed, Pitch and Loudness Superposition & Interference Production, Detection, Dangers Inverse Square Law EM Waves in Communication Law of Reflection

Light & Mirrors. Reflection in Communication.

Refraction & Snell’s Law. Light, Lenses & Total

Internal Reflection The EM Spectrum The Wave Equation Graphing Waves

The World

Communicates

1. Waves

2. Sound

Waves

3. Electromagnetic

Waves

4. Reflection

& Refraction

5. Digital

Communication

& Storage

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1. THE NATURE OF WAVES

Waves Carry Energy

... or in 2 dimensions:

Ripples spreading on the surface of a pond.

...or in 3 dimensions,

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

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.

Pulses moving along a slinky spring

CCoommpprreesssseedd sseeccttiioonnss iinn tthhee sspprriinngg mmoovvee aalloonngg iitt lliikkee aa ““MMeexxiiccaann W

Waavvee””... eenneerrggyy iiss ttrraannssffeerrrreedd,, bbuutt tthhee ccooiillss mmeerreellyy oosscciillllaattee bbaacckk aanndd ffoorrtthh aanndd ddoo nnoott aaccttuuaallllyy ggoo aannyywwhheerree..

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Waves carry energy, without the transfer of matter.

This can occur in 1 dimension:

Describing Waves

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.

Consider a wave in a rope which has been given a single up-and-down “twitch”:

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.

CCRREESSTT

A PULSE WAVE ppaarrtt ooff tthhee rrooppee ((mmeeddiiuumm)) vviibbrraatteess uupp && ddoowwnn

TTRROOUUGGHH EEnneerrggyy mmoovveess

aalloonngg tthhee rrooppee

rrooppee

A PERIODIC WAVE

TTRROOUUGGHH CCRREESSTT EEnneerrggyy mmoovveess

MECHANICAL WAVES require a medium to travel through.

ELECTROMAGNETIC WAVES do not.

A PULSE WAVEis a single wave disturbance.

PERIODIC WAVES contain a series of pulses, with a continuous set of crests and troughs.

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KCiC Physics 1 World Communicates

Site Licence Conditions only

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Rope vibrates up and down A PERIODIC, TRANSVERSE WAVE

TTRROOUUGGHH CCRREESSTT

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.

TRANSVERSE WAVES

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

Energy flow

Vibration in medium

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.

EEnneerrggyy mmoovveess

LONGITUDINAL WAVE IN A SPRING

ccoommpprreessssiioonn

iinn sspprriinngg ((wwhheerree sspprriinngg iissrraarreeffaaccttiioonn ssttrreettcchheedd)) SSpprriinngg vviibbrraatteess

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

LONGITUDINAL WAVES

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

Energy flow

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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).

The Greek letter “lambda”

λλ

is used as the symbol for wavelength.

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) 1 Hz = 1 wave per second.

Wave Measurements

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

their:-Period

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

Note that there is a simple relationship between

Frequency and Period... they are reciprocals.

Velocity

(v) = the speed of the wave,

in metres/sec.(ms-1) There is a simple relationship between Velocity, Wavelength and Frequency:

Velocity = Frequency x Wavelength

THE WAVE EQUATION

V = f

λλ

WAVELENGTH

AMPLITUDE Wave cycles per second

is FREQUENCY

T = 1 and f = 1

f T

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Slide 7 Usage & copying is permitted according to the Site Licence Conditions only

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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

a) V = f λλ 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.)

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)

Wave Equation Calculations

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.

Solution

a) f = 1 / T = 1 / 0.002

= 500Hz (i.e. 500 vibrations per sec.) b) V = f λλ

330 = 500 x λλ

λλ = 330 / 500

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Slide 8

Usage & copying is permitted according to the Site Licence Conditions only

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).

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.

Cork bobs up and down Ripples 00..22 00..66 11..00 11..22 00 -33 ++ 33 DD iiss ppll aacc eemm eenn tt ((cc mm )) O Onnee ppeerriioodd = = 00..88 ss 00..44 00..88 TTiimmee ((ss)) ®

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. λλ = 0.45m)

V = f x λλ

= 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

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Longer

Wavelength FrequencyLower

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”.

You would have found...

INCREASING

DECREASE in

the FREQUENCY

WAVELENGTH

and

DECREASING

INCREASE in

the FREQUENCY

WAVELENGTH

(If VELOCITY is the same)

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Relationship Between Wavelength & Frequency

Shorter

Wavelength HigherFrequency

To have the same speed, the

shorter waves must vibrate at a

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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?

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Wave P

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KCiC Physics 1 World Communicates Slide 11 Usage & copying is permitted according to the ®

Sound Waves

Sound waves are Mechanical(they need a medium)

and Longitudinal(vibrate back-and-forth in the line of the energy flow)

SOUND Energy moves

WAVES

Particles vibrate

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.

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

Velocity of Sound

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.

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

In liquids and solids, sound travels much faster...

...about 1,500ms-1in water

...about 5,000ms-1_{in most metals.}

FREQUENCY = “PITCH”

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

Low Frequency = Low Pitch High Frequency = High Pitch

AMPLITUDE = LOUDNESS or VOLUME

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

Larger Amplitude = Louder Sound Smaller Amplitude = Quieter Sound

2. THE PROPERTIES OF SOUND WAVES

Sound Travels

CCoommpprreessssiioonn CCoommpprreessssiioonn

RRaarreeffaaccttiioonn RRaarreeffaaccttiioonn

CCoommpprreessssiioonnss.. HHiigghheerr aaiirr pprreessssuurree

RRaarreeffaaccttiioonn.. LLoowweerr pprreessssuurree

DD iiss ppll aacc eemm eenn tt ff rroo mm tthh ee eeqq uuii lliibb rriiuu mm TTiimmee The back-and-forth vibration of the medium produces a typical wave shape if graphed.

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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.

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).

““SSqquueeaakkss”” ooff ssoouunndd

EEcchhooeess ffrroomm iinnsseecctt

SONAR

SOund Navigation And Ranging BAT

Humans have invented SONAR technologies for things such as “depth sounding” and detecting

underwater objects... fish or submarines, it all works the

same way.

USES OF

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

AAnnttii-SSuubbmmaarriinnee W Waarrffaarree D Deepptthh SSoouunnddiinngg ®

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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.

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”.

AAdddd ppoossiittiivvee && nneeggaattiivvee ddiissppllaacceemmeennttss

aatt tthhee cciirrcclleedd ppooiinnttss _{wave A}

wave B

Displacement

Resultant

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

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.

TToo ffiinndd aa ““rreessuullttaanntt””,, aadddd tthhee

ddiissppllaacceemmeennttss ooff AA&&BB aatt ccoonnvveenniieenntt

ppooiinnttss ((cciirrcclleedd)) DD iiss ppll aacc eemm eenn tt “resultant” A+B wave A wave B

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Activity 2

SOUND WAVES

Student Name ...

1. Describe a sound wave using 2 technical words.

2. a) If you listened to sound waves of different frequency,

how would they sound different?

b) If you listened to sound waves of different amplitude,

how would they sound different?

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

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.

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DD iiss ppll aacc eemm eenn tt wave A wave B wave C wave D Displacement

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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.

MEMBERS OF THE EM SPECTRUM

Radio (and TV) waves

microwaves infra-red (heat radiation) visible LIGHT ultra-violet X-rays Gamma rays

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.

3. ELECTROMAGNETIC WAVES

W avelength decreasing very short v .long Frequency increasing very high low

Production of EM Waves

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

For example....

Radio wavesare produced by electric currents running back-and-forth in a conducting wire.

Infra-red wavesare 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.

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

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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...

When radio waves hit a suitable aerial wire or antenna, they cause some electrons in the metal to oscillate back-and-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. When the fat

lady sings...

Antenna

Detection & Reception of EM Waves

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. 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.

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.

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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.

EEaarrtthh’’ss ssuurrffaaccee SSuunn

ozzon e layye

r

uuppppeerr aattmmoosspphheerree XX-rraayy && ggaammmmaa

UUVV

ssoommee rreefflleecctteedd

rraaddiioo iinnffrraarree_{dd && lliigg}

hhtt

Danger of High Frequency EM Waves

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

The Sun only produces small

amounts of the even more dangerous X-rays and gamma radiation. Once again, most is absorbed in the upper

atmosphere, this time by ordinary oxygen and nitrogen gases.

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

UV Rays Oxygen O₂₂ does not Absorb UV Ozone O₃₃ Absorbs UV

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®

keep it simple science _{As any form of radiation spreads out from its source its intensity gets less.}

The Inverse Square Law

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.

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.

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

This diagram explains why:

Intensity αα 1 (distance)2 I αα 1 d2 “αα” means “proportional to” xx lliigghhtt ssoouurrccee ddiissttaannccee ““dd”” ddiissttaancee “ 22dd” 22xx SSqquuaarree AArreeaa xx2 SSqquuaarree wwiitthh ssiiddeess ttwwiiccee aass lloonngg..

AArreeaa == 44xx2

SSaammee aammoouunntt ooff lliigghhtt ffaallllss oonn 44 ttiimmeess tthhee aarreeaa

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Activity 3

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?

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Slide 20 Usage & copying is permitted according to the

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Radio & Microwaves

carry radio

and TV broadcasts, telephone

long-distance links, mobile phone

networks, and satellite links for

telephone (including internet) and TV.

If you have “Satellite TV”, the “dish”

on your roof is an antenna to receive

microwaves directly from an orbiting

satellite.

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.

What’s special about

LASER LIGHT?

•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.

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

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

EM Waves & Communication

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

Light

is being increasingly used in the form

of LASER beams carried in optical

fibres for telephone and internet

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)

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.

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.

D Diiggiittaall ssiiggnnaall D Diiggiittaall 11 00 11 11 00 11 ddaattaa W Waavvee ppuullsseess oonn aanndd ooffff

This diagram compares the effect of

AM, FM & Digital Modulation

on the same “carrier wave”

WAVE

MODULATION

Pulse Modulation

(Digital)

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. Optical Fibres carry Pulse Modulated laser beams ““CCaarrrriieerr w waavvee”” N Noo iinnffoorrmmaattiioonn ccaarrrriieedd AAMM ssiiggnnaall AAmmpplliittuuddee cchhaannggeess.. FFrreeqquueennccyy ccoonnssttaanntt FFMM

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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?

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.

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

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.

SOUND

ELECTRICITY

RADIO MICROWAVE

RADIO

SOUND

(or LASER LIGHT)

ENERGY CHANGES

5. Your friend’s phone receives the RADIO signal,

amplifies it as an ELECTRICAL signal and this is

converted to SOUND waves in

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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.

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.

Discussion:

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Activity 4

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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www.keepitsimplescience.com.au Slide 25 ®

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:

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.

4. REFLECTION & REFRACTION

Example: Light waves travelling in air, then hitting glass.

AAbbssoorrbbeedd eenneerrggyy bbeeccoommeess hheeaatt

ABSORPTION of the energy

REFLECTION (bounces off)

TRANSMISSION into the new medium, with possible

REFRACTION (change of direction)

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

i

o

_{= r}

o

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?

The Law of Reflection is still obeyed, as shown:

“Normal” line IInncciidd eenntt rr_aayy RReefflleecc tteedd rraayy ioo roo Reflective surface such as a mirror

The Incident rays P,Q & R are parallel. Each obeys the Law of Reflection, but the

reflected rays go in different directions. The “Normal” for each ray is a dotted line.

P Q R

Uneven, rough surfaces don’t give “shiny” reflections because the light is scattered in all directions.

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®

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

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.

Focus

Reflection of Light from Curved Mirrors

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

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 “blind-spot”. (BUT things look smaller. This can confuse a driver into thinking that other cars are further away.)

“Virtual” Focus

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KCiC Physics 1 World Communicates Slide 27 Usage & copying is permitted according to the ®

Wave reflection from the ionosphere can help with long distance radio communications. It works best with the longer wavelength AM signals.

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. Transmitter Ionnoss pphheerree llaayyeerr Receiver EARTH

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.

M Miiccrroowwaavvee bbeeaamm ttrraavveellss bbeettwweeeenn rreellaayy

ssttaattiioonnss

YYoouurr ssaatteelllliittee TTVV ddiisshh iiss aa rreefflleeccttoorr ttoooo

Microwave Reflector Dishes

TTrraannssmmiitttteerr

ddiisshh RReecceeiivveerrddiisshh

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

• The velocity increases: wave speeds up • The wavelength gets longer.

• Wave refracts away from the normal.

In this case,

i

o

< r

o

io r

o

Incident

Ray _Glass _Air Refracted

Ray

nnoorrmmaall

Refraction

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) • The velocity slows.

• The wavelength gets shorter.

• The beam changes direction towards the normal.

i

o

> r

o

io ro Incident Ray Refracted Ray Air Glass nnoorrmmaall

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.

Angle of Incidence

Angle of Refraction

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Slide 29 Usage & copying is permitted according to the

®

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.

When you graph the angles the result is a curve.

This is not much use for defining any relationship that may exist.

Angle of refraction, roo An gl e of i nc id en ce , i oo

Snell’s Law

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.

The gradient of the 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.

This special ratio is known as the

“REFRACTIVE INDEX” (n)

This is now called Snell’s Law:

Sin roo Si n i oo GGrraaddiiee nntt == rr iissee == SSiinn ii rruunn SSiinn rr

Sine (angle incidence) = velocity (medium 1) = n Sine (angle refraction) velocity(medium 2)

Sin i = V₁ = ₁n₂ Sin r V₂

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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.

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

Refractive Index

(air -> glass) _a

n

_g = sin45 / sin28 = 1.5

and

Refractive Index

(glass -> air) _g

n

_a= sin28 / sin45 = 0.66

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.

1

n

2

= 1

2

n

1

The spoon appears “broken”

at the surface of the tea due to refraction of the light by which we see it. 45oo 45oo 28oo 28oo Refraction air ->> glass Refraction glass ->> air nnoorrmmaall glass

3 beams of light being refracted through a perspex block.

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®

Snell’s Law

Sin i = V₁ = ₁n₂ Sin r V₂

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 3.00x108ms-1, find the velocity in the glass.

Solution

a) Sin i = n sin 35 / sin r = 1.50 Sin r

sin r = sin 35 /1.50 = 0.38238 therefore, angle of refraction, r = 22.5o

b) V₁ = n 3.00x108/ V₂= 1.50 V₂

V₂= 3.00x108/ 1.50 therefore, velocity in glass, V = 2.00x108ms-1

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®

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

Total Internal Reflection

& the Critical Angle

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 Sin i = _gn_a Sin r

so at the

critical angle Sin c = _gn_a Sin 90

and sin 90o= 1, so... Consider the situation when waves are going from

a more dense medium into a less dense medium, such as light going from glass into air.

The waves refract away from the normal.

Now think about increasing the incident angle as shown in this series of diagrams. ioo roo 1 ggllaassss aaiirr ioo r oo 2 bigger i, bigger r ioo_=coo r oo_{= 90}oo 3 CCrriittiiccaall AAnnggllee Sin c = _gn_a= 1 1 _an_g

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®

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

“TOTAL INTERNAL REFLECTION”

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.

If ioo_{> c}oo

the ray cannot get out. It reflects back inside

the glass. ioo_>coo Ray reflects inside glass 4

What Happens Beyond the Critical Angle?

This diagram follows on

from the previous

slide.

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 _{> c}o_{) so Total Internal}

Reflection occurs and the beam stays totally within the fibres over long distances.

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

Optical fibre llaasseerr bbeeaamm Lower index outer layer. Core. High index.

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Activity 5

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?

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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”.

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.

5. DIGITAL COMMUNICATION & DATA STORAGE

®

A simple list of some of the technologies involved is: 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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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 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.

TECHNOLOGY CASE STUDY:

GLOBAL POSITIONING SYSTEM

(GPS)

GPS

By doing the same for 2 other satellites, the GPS unit rapidly “triangulates” the signals from 3 satellites to pin-point 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.

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.

Satellite orbits Satellite 1 SSaatteelllliittee 22 Satellite 3 GGPPSS rreecceeiivveerr Earth