keep it simple science tm emmaus catholic college sl...

keep it simple science TM

www.keepitsimplescience.com.au

Emmaus Catholic College SL#802440

1

but first, let�’s revise...

Preliminary Physics Topic 1

THE WORLD COMMUNICATESWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. THE NATURE OF WAVES

2. THE PROPERTIES OF SOUND WAVES3. ELECTROMAGNETIC WAVES

4. REFLECTION & REFRACTION5. DIGITAL COMMUNICATION & DATA STORAGE

...in the context of communications

ENERGYEnergy 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).

Some forms of energy move around as WAVES. Awave is a carrier of energy. In a wave, energy moves,but matter does not. For example, compare these 2situations:

When the wind blows, both matter (the air)and energy move....

THIS IS NOT A WAVE!

In this case, only the energy really moves. The air only vibrates as the SOUND WAVES pass.

Waves Carry EnergyWithout the Transfer of Matter

TYPES of WAVESExamples of energy which moves around as waves include

�• SOUND

�• LIGHT

�• RADIO SIGNALS

�• WATER WAVES

�• X-RAYS

�• MICROWAVES

... and many more

PhotoHelen Lee

ENERGY CONVERSIONSEnergy can be converted from one form to another.

In your mobile phone the SOUND WAVES of your voiceare converted to ELECTRICAL signals then transmitted asRADIO WAVES to your friend, whose phone converts itback 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.

PhotoPhilipp Pilz

Preliminary Physics Topic 1copyright © 2005-2007 keep it simple science


www.keepitsimplescience.com.auPreliminary Physics Topic 1copyright © 2005-2007 keep it simple science


2

�•mechanical�•EM waves�•transverse�•longitudinal �•wavelength

�•amplitude�•frequency�•period

Light & MirrorsReflection inCommunication

Total InternalReflection

& Critical Angle

GeneralTypes

of Waves Graphing Waves

(displacement-ttime)

Velocity

Frequency & PitchAmplitude & Loudness

WaveEquationV = f!!

Natureof Sound

Waves Principleof

Superposition

Dangers ofEMR

Production&

Detectionof EMR

TheEMR

Spectrum

Inverse SquareLaw

I "" 1 d2

RefractionSnell�’s Law

Sin i = v1 = nSin r v2

EMRin

Communication

TTHHEE WWOORRLLDDCCOOMMMMUUNNIICCAATTEESS

The Natureof

Waves

Propertiesof

SoundWaves

ElectromagneticWaves(EMR)

Reflection&

Refraction

DigitalCommunication

&Data Storage

CONCEPT DIAGRAM (�“Mind Map�”) OF TOPICSome students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts andimportant facts. As you proceed through the topic, come back to this page regularly to see how each bit fits thewhole. At the end of the notes you will find a blank version of this �“Mind Map�” to practise on.

WaveMeasurements

Lawof

Reflection




3

Waves Carry Energy

Waves carry energy, without the transfer of matter.

This can occur in 1 dimension:

... or in 2 dimensions:

Ripples spreading on thesurface of a pond.

...or in 3 dimensions,

such as when light radiates in all directions from a glowingobject.

Waves & Mediums

Mechanical waves are those which need a �“medium�” totravel through. For example, a water wave must have waterto travel in. Sound waves need air, or water, or somesubstance 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 fastestin a vacuum. EM waves include light, radio waves, ultra-violet and other types, and are studied in detail in a latersection.

Describing Waves

A wave is a vibration. In a mechanical wave, the �“particles�”(atoms & molecules) in the medium vibrate to transmit thewave energy. In EM waves the vibration occurs in electricand 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 goanywhere. Particles of the �“medium�” (the rope fibres)vibrate up-and-down as the energy moves across.

This form of a wave, where the medium vibrates at rightangles to the direction that the energy moves, is called aTransverse wave.

If the rope is wiggled constantly up-and-down, you get notjust one pulse, but a periodic wave with one pulse followinganother.

TRANSVERSE Energy flow

WAVESVibration in medium

1. THE NATURE OF WAVES

Pulses moving along a slinky spring

Compressed sections in the spring move along it like a �“MexicanWave�”... energy is transferred, but the coils merely oscillate backand forth and do not actually go anywhere. CRESTA PULSE WAVE part of the rope (medium)

vibrates up & down

TROUGH

Energy movesalong the rope

rope

AA PPEERRIIOODDIICC,, TTRRAANNSSVVEERRSSEE WWAAVVEE

WAVELENGTH

AMPLITUDE

AMPLITUDETROUGH

CREST

Energy moves

TRANSVERSE WAVES

THE VIBRATION OF THE WAVEIS AT RIGHT ANGLES

TO THE DIRECTION OF ENERGY FLOW

PhotoPhilippPilz




4

Longitudinal waves are when the particles of the mediumvibrate back-and-forth in the same line as the energymoves. For example, when a series of �“compressions�” and�“rarefactions�” are sent along a slinky spring.

Energy moves

LLOONNGGIITTUUDDIINNAALL WWAAVVEE IINN AA SSPPRRIINNGG

compressionin spring

rarefaction(where spring is

stretched)

Spring vibrates

LONGITUDINAL WAVES

THE VIBRATION OF THE WAVEIS IN THE SAME LINE

AS THE DIRECTION OF ENERGY FLOW

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

The Greek letter �“lambda�” !is used as a symbol for wavelength.

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

Frequency (f) = the rate at which the wave is vibrating.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/sec.

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 completevibration to occur.

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

Velocity (v) = the speed of the wave, in metres/sec.(ms-1)

There is a simple relationship between Velocity, Wavelengthand Frequency:

Velocity = Frequency x Wavelength

TRY THE WORKSHEET, at the end of this section.

THE WAVE EQUATION

V = f!!WAVELENGTH

AMPLITUDE

Energy moves

Wave cycles per secondis FREQUENCY

T = 1 and f = 1 f T




5

Graphing WavesA good way to represent a wave is by using a graph.

Imagine a floating cork bobbing up and down as a series ofripples move across the water surface (i.e. a periodic wave).

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

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

What you CAN read from a Displacement-Time graph:

Amplitude. The vertical scale measures the displacement ofthe cork from the �“equilibrium�” position (i.e. the flat watersurface).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 easilyread 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 couldcalculate the wavelength, or vice versa.

e.g. if the ripples are 0.45m apart: (i.e. ! = 0.45m)V = freq. x wavelength

= 1.25 x 0.45So, velocity = 0.56 ms-1

TRY THE WORKSHEET (next page)

Graphing a Longitudinal WaveYou might think these Displacement-Time graphs wouldn�’twork for a Longitudinal wave where the particles vibrateback-and-forth rather than up-and-down.However, the graph of a longitudinal wave can be exactlythe same... you just have to realise that the �“displacement�”is sideways displacement from the �“equilibrium position�”,instead of up-down.Amplitude, Period and Frequency can all be determined inexactly the same way.

Relationship BetweenWavelength & Frequency

You may have carried out a �“First Hand Investigation�” inclass to see how a change in Frequency (at constantvelocity) affects the wavelength. Maybe you used a slinkyspring, or watched the water waves in a �“ripple tank�”.

You would have found...

INCREASING DECREASE inthe FREQUENCY WAVELENGTH

and

DECREASING INCREASE inthe FREQUENCY WAVELENGTH

(If VELOCITY is the same)

Cork bobs up and down

Ripples

0.2 0.6 1.0 1.2

0-3

+3

DDiiss

ppllaacc

eemmeenn

tt ((cc

mm))

One period= 0.8 s

LongerWavelength

To have the same speed, the shorter waves must vibrate at ahigher frequency

LowerFrequency

ShorterWavelength

HigherFrequency

0.4 0.8

Time (s)

Waves carry a)...................................... without thetransfer of b)................................ �“Mechanical�”waves require a c)............................. to travel in.Examples are d)...................... and ............................�“Electromagnetic�” waves do not need a mediumand can travel in a e).........................................Examples include f)............................... and...................................

A g)................................... wave is when thevibration and the movement of energy areh)........................................................In a Longitudinal Wave, the vibration and theenergy movement are i)..............................................

�• j)........................................ is the distance fromcrest to crest.

�• Amplitude is the k)...................................................

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

4.5 = f x 85f = 4.5 / 85

= 0.053 Hz (5.3 x 10-2 Hz)(i.e. only a 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)

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

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

b) V = f !330 = 500 x !

! = 330 / 500= 0.66m (i.e. 66cm from crest to crest)

6





�• Frequency is the number of l)................................per second. The SI unit is the m)................. (........)

�• n)............................................ is the time for onecomplete vibration. This is theo).................................... of frequency.

�• Velocity is the speed of the wave and is equal top).......................... multiplied by q).............................

On the graph of a wave, showing Displacement vTime, the vertical scale shows ther)..................................... of the wave, while thehorizontal allows you to read the value of thes)........................................ and then easily calculatethe t)......................................................

For waves travelling at the same velocity,increasing the frequency wouldu).................................... (increase/decrease) thev)................................................, and vice-versa.

TRY THESE

1. a) Find the velocity of a sound wave in water if itvibrates 280 times per second and has a wavelength of5.20m.b)What is the period of this wave?

2. An earthquake shockwave travels through rock at avelocity of 2,500 ms-1. Its frequency is 0.400 Hz.What is the wavelength?

3. What is the wavelength of a sound wave with frequency1200Hz? Sound travels in air at 330ms-1.

4. An ocean water wave in deep water travels at a velocityof 6.50ms-1. Its period is 16.0s.a) What is the frequency?b) Wavelength?c) As the wave enters shallower water it keeps the samefrequency but slows down to only 2.20 ms-1. Whathappens to the wavelength?

continued...

COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

Worksheet 1Part A Fill in the blank spaces

Part B Practice ProblemsWave Measurements and the Wave Equation

Remember that for full marksin calculations, you need to show

FORMULA, NUMERICAL SUBSTITUTION,APPROPRIATE PRECISION and UNITS

Worksheet 1 continued

5.a) Red light has a wavelength of 7.00x10-7m, and travels at3.00x108ms-1. What is the frequency?b) Blue light has a wavelength of 3.00x10-7m and travelsat the same speed. What is the frequency?

6.Radio signals travel at the speed of light.(3.00x108ms-1) A radio station has a frequency of530 kHz (=530,000Hz).a) What is the period of the waves?b) What is the wavelength?

7.In World War II, one major technological advance was thedevelopment of �“centimetric RADAR�”... Radar thatworked on radio waves of only a few centimetres inwavelength. (Previous radars worked at wavelengths of ametre or longer)Compare the frequency of a radio wave 2.50m long, withone 2.50cm long.(Assume they both travel at the speed of light)

Part C Wave Graph Exercise

The following sketch graph shows 3 different waves �“P�”,�“Q�” and �“R�”. For each wave;

i) What is the Amplitude?ii) State the (approx) displacement at time t=0.03siii) What is the Period of each wave?iv) What is the Frequency of each wave?v) Given that wave �“P�” has a wavelength of0.50m, calculate its velocity.

vi) Waves �“Q�” & �“R�” both travel with a velocityof 9.5ms-1. Find their wavelengths.

7







8.When a guitar string is plucked, a wave vibration runsback and forth through the string. The string is 0.96mlong and it is found that exactly 8 complete wavelengthsfit along the string at a time. The vibration frequency is384Hz.How fast do the waves travel through the string?

9.X-rays are very short wavelength EM waves which travelat the speed of light. If the wavelength is 1.50x10-11 metre,a) find the frequency.b What is the period of the X-rays?

PQ

R0.1

0.1

-00.1

0

0.05

Time (s)

Dis

plac

emen

t (m

)




Sound WavesSound waves are Mechanical (they need a medium)

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

SOUND Energy movesWAVES

Particles vibrate

Instead of crests and troughs, a series of �“compressions�”and �“rarefactions�” pass through the medium as a soundtravels. The atoms and molecules are alternately �“squashedtogether�” and then stretched apart as the energy flowsthrough.

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

Velocity of SoundSound travels at different speeds in different mediums.In air, sound travels at about 330-350 ms-1, (about 1,200km/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-1 in water...about 5,000ms-1 in most metals.

FREQUENCY = �“PITCH�”

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

Low Frequency = Low PitchHigh Frequency = High Pitch

AMPLITUDE = LOUDNESS or VOLUME

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

Larger Amplitude = Louder SoundSmaller Amplitude = Quieter Sound

8


2. THE PROPERTIES OF SOUND WAVES

Sound Travels

Compression CompressionRarefaction Rarefaction

Compressions. Higher air pressure

Rarefaction. Lower pressureDis

plac

emen

t fro

mth

e eq

uilib

rium

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).

�“Squeaks�” of sound

Echoes from insect

SONAR SOund Navigation And Ranging

BAT

Humans have invented SONAR technologies for thingssuch as �“depth sounding�” and detecting underwaterobjects... fish or submarines, it all works the same way.

The time delay betweensending a sound �“ping�’ and

receiving the echo, givesdepth and distance

�“FishFinders�”

DepthSounding

Anti-submarineWarfare

USES OFSONAR

Time




9

The Principle of SuperpositionAll waves have the ability to pass through other waveswithout being affected. For example, you could shine a redspotlight across a beam of blue light, and each colour andbeam will emerge on the other side exactly the same.

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

Very simply, the displacement of the two waves addtogether 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 wasconstructive interference... the resultant has an amplitudewhich is the sum of A+B.

However, if the waves are �“out of phase�” (for example, ifcompression coincided with rarefaction) then there isdestructive interference... the opposite amplitudes maycancel each other out.

Theoretically, if 2 sound waves had the same amplitude andwere 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 timeperiods to give �“interference patterns�” and �“beat sounds�”.

To find a�“resultant�”, addthe displace-

ments of A&B atconvenient

points (circled)

Dis

plac

emen

t

Add positive &negative displacements

at the circled points wave A

wave BDis

plac

emen

t

When an in-coming wave meets areflected wave going back out, their

amplitudes add together.

The Principle of Superposition

Technically, a breaking wave is not a wave at all.Once it breaks, the water begins moving forward (which allows you to

catch it) and so both energy and matter are flowing forward...this is NOT a wave!

True water waves are the �“swells�” which you cannot catch.

Photo: Chris Potter

PPhhoottoo:: IIggoorr KKaassaalloovviicc

�“resultant�” A+B

wave Awave B

Resultant



www.keepitsimplescience.com.au10


Worksheet 2

Part A Fill in the blanks. Check your answers at the back

Sound waves are a)....................................... and b).................................... A sound wave consists of a series of high pressurec)................................................... and lower pressure d)........................................................... travelling through the medium.In air, the speed of sound is about e)............. ms-1, but it is much f).................................. (higher/lower) in water or in solidssuch as metals.

The �“pitch�” of a sound is related to the g).................................... of the wave. The amplitude of the wave determines theh)............................. of the sound we hear. �“Echoes�” occur when sounds i)............................................. Some animals use echoesfor j)............................................ Humans use the technology of k)............................................ for �“depth sounding�” andl)..............................................When 2 or more waves coincide, they will interfere with each other. The m).................................................. wave can be foundby adding together the separate wave n)....................................................

Part B Principle of Superposition ExerciseFind the resultant of these 2 waves by adding the displacements at the circled points, then join the “sum” points with an even curve.


-ve

Dis

plac

emen

t

+

ve

IF THESE ARE SOUND WAVES, DESCRIBE WHAT YOU WOULD HEAR (check your answer at the back )

EM WavesElectromagnetic waves are Transverse waves which doNOT require a medium to travel through...they travel through a vacuum at 3.00x108ms-1, the �“speedof light�”. They can travel through many other substancesat slightly slower speed. For example, light can travelthrough glass or water at speeds of around 2.5x108ms-1. Inair, the speed is so close to the speed in a vacuum that, forsimplicity, (K.I.S.S. Principle) we take it to be the same.

EM radiation (EMR) does not require a medium becausethe waves propagate as vibrations of electric and magneticfields, not as vibrating particles.

MEMBERS OF THE EMR 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 ofradiation, you must realise that they are really all the samething, just at different wavelengths and frequencies.

Production of EM WavesAll EM waves are produced in basically the same way:vibration or oscillation of electrically charged particles.For example....Radio waves are produced by electric currents runningback-and-forth in a conducting wire.

Infra-red waves are made by molecules vibrating rapidlybecause of the heat energy they contain.

Light is emitted when electrons rapidly �“jump�” down froma higher to a lower orbit around an atom.

Gamma waves come from the vibrations of chargedparticles within an atomic nucleus, during a nuclearreaction in the atom.

Detection & Reception of EM WavesJust 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 strikesomething and are absorbed, they may cause �“sympathetic�”vibrations within it.

In cartoons and the movies (not in real life) the operasinger hits a high note and all the wine glasses begin tovibrate and then shatter... a fictional example of resonance.

Some real examples...

When radio waves hit a suitable aerial wire or antenna, theycause some electrons in the metal to oscillate back-and-forth �“in sympathy�” with the wave. These oscillations areamplified electronically and the signal converted to soundin the speaker, allowing you to listen to the radio.

When infra-red waves hit your skin they cause certainmolecules to begin to resonate and vibrate. This sets offnerve messages to the brain and you feel warmth or heaton your skin.

In a camera the light causes resonance in chemicals in thefilm. Chemical reactions occurwhich permanently alter the filmso that an image appears when�“developed�” later. Different filmcan be sensitive to infra-red,(photos in the dark) or X-raysfor medical uses.

11


3. ELECTROMAGNETIC WAVES




Wav

elen

gth

decr

easi

ngve

ry s

hort

v.lo

ng

Freq

uenc

y in

crea

sing

very

hig

hlo

wWhen the fat lady sings...



12

Danger of High Frequency EMRHigh frequency EMR (ultra-violet, X-ray & gamma) can bevery dangerous to living things.

A little UV gives you a suntan, but long-term exposureleads to skin damage, premature skin �“ageing�”, and isimplicated in causing skin cancers, including melanoma.

The Sun produces dangerous quantities of UV radiation,but luckily most of it is absorbed by the �“ozone layer�” inthe upper atmosphere of the Earth.

�“Ozone�” is a form of oxygen which has 3 atoms permolecule (O3) instead of the normal 2 (O2). The ozonemolecules resonate well at the frequency of UV and soabsorb it strongly.

The Sun only produces small amounts of the even moredangerous X-rays and gamma radiation. Once again, mostis absorbed in the upper atmosphere, this time by ordinaryoxygen and nitrogen gases.

Infra-red and light radiation penetrate well, (although about30% is reflected) and while some radio frequencies getthrough, many get absorbed or reflected.

The Inverse Square LawAs any form of radiation spreads out from its source itsintensity gets less. For example, a sound becomes quieter ifyou�’re further from the source, or a light is not so bright asyou move further from it.

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

This diagram explains why:

At distance �“d�” from the light source, some light energyfalls on an area of x2 units. At twice that distance (2d) thesame amount of light would fall on an area of 4x2. Thebrightness of the light must be only 1/4 as much (since thesame amount of light is falling on 4 times the area.)

So, twice the distance 1/4 as bright3 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 brighterat 1/3 the distance, 9 times brighter

...and so on.

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



x

lightsource

distance �“d�”

distance �“2d�”

2x

Intensity "" 1 (distance)2

I "" 1 d2

�“""�” means�“proportionalto�”

UV Rays

Ozone O3Absorbs UV

Oxygen O2does notAbsorb UV

TRY THE WORKSHEET PROBLEMSAT THE END OF THIS SECTION

Earth�’s surface

Sunozone layer

upper atmosphere

X-ray & gamma

UV

somereflected

radio

infrared & light

Square

Area x2

Square withsides twice aslong.

Area = 4x2

Same amountof light fallson 4 timesthe area



13

EMR & CommunicationHumans rely on sound waves for communicating by directspeech, but all our modern communication technologiesrely on EMR.

Radio & microwaves carry radio and TV broadcasts,telephone long-distance links, mobile phone networks, andsatellite links for telephone (including internet) and TV.If you have �“Satellite TV�”, the �“dish�” on your roof is anantenna to receive radio waves directly from an orbitingsatellite.

Add to that, 2-way radio for military uses, CB amateurs andboating, shipping and aircraft communications, and youbegin to realise how many radio waves are zapping around.

Light is being increasingly used in the form of LASERbeams carried in optical fibres for telephone and internetcommunication.

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



What�’s special about LASER LIGHT?

�•It is one, pure frequency of light.�•The waves are all in phase and sothey interfere constructively toform a very intense, tight beam.�•A laser beam will stay inside anoptical fibre and not �“leak�” out ordissipate for long distances.�•A laser can be turned on & offvery rapidly, so it�’s perfect forhigh speed digital communication.

Frequency Modulation (FM)

The amplitude stays constantwhile the frequency (and

wavelength) vary within a fixedrange. The information (voice,

music etc) is �“coded�” in thevariations of frequency.

FM radio gives much betterfidelity and is superior, compared

to AM, for the quality ofsound (eg for music)

received.

Pulse Modulation(Digital)

To carry informationin digital form thewave must switchrapidly between 2

different states, representing the�“1�” and �“0�” of digital codes. Thewave can be switched rapidly onand off (as in the diagram) or

switched back-and-forth betweendifferent �“phase states�”... phase

modulation.

Amplitude Modulation (AM)

The frequency (and wavelength)of the wave stays constant while

the amplitude varies.

The changing amplitude �“codesfor�” the information beingcarried... whether voice or

music, or whatever.

�“Carrierwave�”

AMsignal

FMsignal

Digitalsignal

Digital 1 0 1 1 0 1data

Noinformationcarried

Amplitudechanges.Frequencyconstant

Freq. changes.Amp.constant

Wave pulseson and off

This diagram compares the effect ofAM, FM & Digital Modulation

on the same �“carrier wave�”

Laser Light Show

Photo: Keith Syvinski

WAVEMODULATION




14

Case Study: MOBILE (CELL) PHONESWhen you use a mobile phone, the sound of your voicegoes into a microphone and almost instantly pops outthe other end into your friend�’s ear.What happens in between?

In the future we will need to switch morecommunications to use the laser light / optical fibremethod wherever possible, and to make better use ofthe RF bands. For example, it is possible to use thesame frequency �“channel�” for several differentpurposes as long as the different signals are modulateddifferently and as long as the radio receivers aresophisticated enough to pick out only the desiredsignal and ignore the others.

One thing is for sure... humans will keepcommunicating and the need for new services willkeep expanding. So far, our technology has alwaysmanaged to keep up, and it will probably continue todo so.

Modern communication systems have developedrapidly and new features and capabilities seem to comeout every day. It seems that the entire system isunlimited and that it can continue to expand andimprove forever.

Well perhaps it can, but NOT while continuing to usethe radio end of the EMR spectrum. Each �“station�”or channel must operate on a different frequency orelse 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 andshipping channels, military, police and emergencyservice channels, etc. etc. all using the RF (RadioFrequency) part of the EMR spectrum, that it isbecoming difficult to keep expanding services withoutinterfering with existing channels.

1. The SOUND energy ofyour voice is converted to

ELECTRICAL signals by themicrophone. The electricalsignal is used to digitally

modulate a RADIO wave.

2. The digital RADIO signal istransmitted by your phone and received

by the local �“cell�” antenna.

3. If your call is going to aperson in another location

(a different �“cell�”) thesignal is converted into a

modulatedMICROWAVE and

beamed, via hilltop relaytowers, to the correct area.(Alternatively, it might besent as a modulated Laser

LIGHT beam throughoptical fibres).

4. In the other cell area, the signalis converted back to a modulatedRADIO signal and transmitted.5. Your friend�’s phone receives the

RADIO signal, amplifies it as anELECTRICAL signal and this is

converted to SOUND waves in theirearphone.

SOUND ELECTRICITY RADIO MICROWAVE RADIO SOUND(or LASER LIGHT)

ENERGYY CHANGES

Discussion:LIMITATIONS OF COMMUNICATION CHANNELS

Worksheet 3Part A Fill in the blank spaces

Electromagnetic waves are a)...................................waves which b).............................. (do/do not)require a c).................................... to travel in. Theyall move at the �“speed of light�”, which isd)..................................... ms-1 in a vacuum. Themembers of the EMR spectrum (in order ofincreasing frequency) are:e)..............................., f)..............................................,g)............................, h).........................................,i)................................., j).............................................,and k)...............................................

All EM waves are produced when electricalcharges l)........................................... They are alldetected/received by the process ofm)................................................. This is when thewave is absorbed by a substance and causeselectrons or molecules to n)......................................�“in sympathy�” with the wave.

High frequency EMR, such aso)............................................. is dangerous to life.Luckily, the p)................................... layer of theatmosphere absorbs most of the dangerousq)........................... rays from the Sun.

All forms of radiation decrease in intensity inproportion to the r).......................................... fromthe source, so if distance is doubled, the intensitywill drop to s)................................ (fraction)

EM waves are very important in moderncommunications. The wave types used are mainlyt)................................. and .........................................,but light is being used more and more in the formof u)......................................... carried insidev)....................................... fibres.

Waves carry information by the process ofw).................................. This can be done in 3 ways:�• �“AM�” stands for x)................................................................., in which the information iscarried as fluctuations in they)....................................... of the wave.�• �“FM�” stands for z).............................................................................., in which the signal iscarried by variations in aa)........................................of the wave.�• Digital signals are carried byab)..................................... modulation in which thecarrier wave rapidly switches between 2 states(e.g. on and off).

The energy changes occurring in a mobile phonecall are as follows: Sound waves of your voice areconverted to an ac).........................................signal. This is used to modulate aad)........................................... wave transmitted tothe local �“cell�” antenna. Next, the signal is sentvia ae)......................................... link, or LASERbeam to another �“cell�” station. Here it istransmitted again as a af)......................................signal. The receiving phone converts this to anag)................................... signal and finally it isconverted back to sound waves.

Part B Practice ProblemsThe Inverse Square Law

Example Problem:At a distance of 5m the brightness of a light ismeasured to be 36 units. How bright would itbe if viewed from 15m?

Answer:Since the distance is 3x further, then intensitywill be 1/9. So new brightness = 36/9 = 4units.

TRY THESE:1.At a distance of 10m from a light, thebrightness (intensity) is 48 units. What intensitywould it have at distance:a) 20m?b) 40m?c) 100m?d) 5m?

2.How much stronger would a radio signal be ifyou moved from 100km, to 25km distance fromthe transmitter?

3.At 2m from a flame the brightness is 32 units.At what distance would the brightness be 2units?

4.One light bulb (at a certain distance) gives �“I�”units of light intensity. To get the same lightintensity at double the distance, how manyidentical bulbs need to be switched on?

15






When a Wave Hits a BoundaryWhen a wave is travelling through one medium and thenstrikes a different medium, one of 3 things can happen atthe boundary:

It is quite possible that all 3 things can happen at once. Forexample, if a beam of light is travelling through air, andthen strikes a glass window:

�• the glass ABSORBS some of the light.�• some REFLECTS off the glass�• some is TRANSMITTED through the glass.

ReflectionThe �“Law of Reflection�” is very simple:Whatever angle a �“ray�” of light hits the surface, it willbounce off again at the same angle.

OR, more technically:

Angle of = Angle ofIncidence Reflection

io = ro

The trickiest bit is how the angles are measured. They mustbe measured between the rays and the �“NORMAL�”... animaginary line at right angles to the surface.

What if the Surface Isn�’t Flat?The Law of Reflection isstill obeyed, as shown:

Reflection of Light from Curved Mirrors

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

Concave mirrors can give ENLARGED images if viewedfrom the right distance, such as a household shaving mirroror make-up mirror, which gives a magnified reflection ofyour face. This is also the basis of a �“reflecting telescope.�”

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

Convex mirrors produce smaller (�“diminished�”) images, butgive a wider-angle view. An example of use is the sidemirrors on a car which give you a wide-angle view into thedriver�’s �“blind-spot�”.

16


4. REFLECTION & REFRACTION




The Incident rays P,Q &R are parallel.

Each obeys the Law ofReflection, but thereflected rays go indifferent directions.

The �“Normal�” for eachray is shown as a dotted

line

P QR

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

AAbbssoorrbbeedd eenneerrggyybbeeccoommeess hheeaatt

AABBSSOORRPPTTIIOONNooff tthhee eenneerrggyy

RREEFFLLEECCTTIIOONN((bboouunncceess ooffff))

TTRRAANNSSMMIISSSSIIOONN iinnttoo tthheenneeww mmeeddiiuumm,, wwiitthh ppoossssiibbllee RREEFFRRAACCTTIIOONN

((cchhaannggee ooff ddiirreeccttiioonn))

�“Normal�”line

Incident ray

Reflecte

d ray

iooroo

Reflectivesurfacesuch as amirror

FFooccuuss

�“�“VViirrttuuaall�”�”FFooccuuss

This is why uneven, rough surfaces don�’t give �“shiny�” reflections.

Light is scattered in all directions.



17

Reflections in CommunicationsWave reflection from the ionosphere can help with longdistance radio communications. It works best with thelonger wavelength AM signals.

The Ionosphere is a zone in the upper atmosphere wherethe air molecules are partly ionized (electrically charged) by radiations from the Sun. The ionized gases act as areflective surface to radio waves of certain wavelengths.

TV signals and FM (shorter wavelengths) radio do notreflect so well and generally you need to be in �“line ofsight�” from the transmitter to get good reception.

Another example involves how Microwaves are transmittedand received. Microwaves are used to relay TV programs toregional transmitters and to relay long distance phone calls(including internet) from city to city.

At the transmission end, a curved reflector keeps the wavesin a tight beam aimed at the next relay station. The receiverhas a similar dish to focus the waves into the receivingantenna.

RefractionWhen waves enters a new medium several things happen:

�• the waves change their speed (get faster or slower)�• their wavelength changes (gets longer or shorter)

Note that frequency does NOT change!�• they change direction (unless they hit at 90o to surface)

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

�• velocity slows�• wavelength shorter (but frequency remains unchanged)�• refracts towards the normal.

io > ro

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

�• velocity speeds up�• wavelength gets longer�• refracts away from the normal.

io < ro

When a light ray refracts, its wavelength changes, butfrequency stays the same. Since COLOUR is

determined by frequency, there is no colour changeduring refraction.



Transmitter

Ionospherelayer

Receiver

Microwavebeam travelsbetween relaystations

Your satellite TV dish is a reflector too

Microwave Reflector Dishes Angle ofIncidence

Angle ofRefraction

ReceivingAntenna

ReflectorDish

ioro

Incident Ray

Refracted Ray

Air Glass

ioro

Incident Ray

Glass Air

Refracted Ray

normal

normal

PhotoHelen Lee

EARTH

Transmitterdish

Receiverdish



18

Snell�’s LawYou may have carried out an investigation in class using a�“Ray Box Kit�” to measure angles of incidence and anglesof refraction.

When you graph the angles the result is a curve.

This is not much use for defining any relationship that mayexist.

In 1621, Snell discovered that if you graph the Sine ratiosof the angles, the points lie in a straight line. You may havedone the same with your experimental data.

The fact that it�’s a straight line means there is a directrelationship between Sin i and Sin r.

The gradient of the line is not only the ratio between theSine of the angles, but is also equal to the ratio of velocitiesof the wave in the 2 mediums involved. This special ratiois known as the �“REFRACTIVE INDEX�” (n)

This is now called Snell�’s Law:

Refractive IndexWhen waves enter a new medium, and then exit it again, therefractions that occur on the way in, are the opposite ofwhat happens on the way out.

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

Refractive index (air -> glass) ang = sin45 / sin28 = 1.5andRefractive index(glass -> air) gna = sin28 / sin45 = 0.66

These 2 values are RECIPROCALS !! ...and this willalways be the case... the index of refraction going in is thereciprocal of the index coming out.



Angle of refraction, ro

Angl

e of

inc

iden

ce, i

o

Sin ro

Sin

io

Gradien

t = ris

e = Sin

i

run Sin

r

TRY THE WORKSHEET at the end of this section

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

Sin i = V1 = 1n2Sin r V2

1n2 = 1 2n1

3 beams of light beingrefracted through aperspex block.

Pencil appears �“broken�”at the water surface, dueto refraction of the lightby which we see it.

4455o

4455o

2288o

2288o

Refractionair -> glass

Refractionglass -> air

normal



19

Total Internal Reflection & the Critical Angle

Consider the situation when waves are going from a moredense medium into a less dense medium, such as lightgoing from glass into air.

The waves refract away from the normal.

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

There comes an angle (called the �“Critical Angle�”, (c))where the angle of refraction = 90o. At this point therefracted ray runs along the edge of the glass, but does notcross the boundary.

So, when the angle of incidence io = co, then ro= 90o

# Sin c = gna Sin 90

and sin 90o = 1, so...

TRY THE WORKSHEET at the end of this section

... But What Happens Beyond the Critical Angle?At incident angles larger than �“c�”, the ray reflects backinside the glass... this is called

�“TOTAL INTERNAL REFLECTION�”

This has one very important application in communicationtechnology...

Optical fibres are thin strands of very pure glass that cancarry communications signals in the form of laser lightbeams. The laser beams stay within the fibres because oftotal internal reflection.

Each fibre is a core strand of glass, with another layerwrapped around it. The outer layer has a lower refractiveindex than the core, so even where the fibre bends arounda corner, the laser light will generally strike the boundary atan incident angle greater than the critical angle.

Whenever the laser beam hits the boundary between the 2layers, the angle of incidence exceeds the critical angle,(io > co) so Total Internal Reflection occurs and thebeam stays totally within the fibres over long distances.

(If the fibre is �“kinked�” at a really sharp angle this won�’twork... the fibres need to bend around corners inreasonably gentle curves.)



Sin c = gna = 1 1 ang

This means that the Sine ratio of thecritical angle �“C�” is equal to

the reciprocal ofthe refractive index of the glass.

If io > co

the ray cannot get out,but reflects back insidethe glass

Core.highindex

lower indexouter layer

Laser �“bounces�” around cornersby total internal reflection

Optical fibre

iiorro

1

glass

air

iiorro

2

bigger i,bigger r

iio==cco

rro== 9900o

3

Critical Angle

iio>>cco

RRaayyrreefflleeccttssiinnssiiddeeggllaassss

4

laser beam

Worksheet 4

Part A Fill in the blanks. Check answers at the back.

When a wave meets the boundary between one mediumand another, any of 3 things can occur: the wave�’s energycan be absorbed, or the wave can bea).......................................... or ....................................................

The Law of Reflection simply states that the angle ofb)........................... equals the angle of c)..................................The angles must be measured from the wave �“ray�” to thed)......................................, which is an imaginary line which ise).................................................. to the boundary. Concavemirrors reflect light into a f).............................. point and canproduce enlarged images, such as in a reflecting telescope.A g)........................................... mirror reflects light outwards.This produces images which are h)........................................,but have a wider field of view. A practical use for thismirror is i)......................................................................................In communications, reflection is useful for long-distanceradio reception. Some radio wavelengths reflect from thej).................................................... layer in the upperatmosphere, and are �“bounced�” around the curvature ofthe Earth. Satellite �“dishes�” and k)........................................antennas use reflection to focus wave signals into thereceiver.

Refraction occurs when waves go from one medium intoanother. The waves may change in l)......................................,and ................................. and .......................................................For example, when light goes from air into glass its speedm)......................................, and its n)..........................................gets shorter (although o)............................................ does notchange) It also changes direction, goingp)......................................... the normal.

Snell�’s Law describes the direct relationship between theSine ratios of the angles of q)...................................... and............................................ This ratio is called ther)......................................................... It is also equal to theratio between the s)........................................ of the wave inthe 2 different mediums. The index for the wave enteringthe medium, and the index for the wave exiting the mediumare always t)............................................ of each other.

When a light ray is going from a �“slower�” medium into a�“faster�” one, the ray will refract u).........................................the normal. As the angle of incidence increases, so will theangle of refraction, until the refracted rayv).................................................................of the boundary.The angle of incidence at which this happens is called thew)....................................... angle. At angles of incidencegreater than this angle, x)..................................................................................... occurs, and the ray stays within the�“slower�” medium. This property is used in optical fibretechnology to ensure that y).............................................beams stay within the fibres.

Part B Practice Problems

Snell�’s Law Sin i = V1 = 1n2Sin r V2

Example ProblemA beam of light goes from air into a glass block with arefractive 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 inthe glass.

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

sin r = sin 35 /1.50= 0.38238...

therefore, angle of refraction, r = 22.5o

b) V1 = n 3.00x108 / V2 = 1.50V2

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

TRY THESE

1. In an experiment, a student sent a beam of light into ablock of clear plastic. The angle of incidence was measuredas 50o. The angle of refraction was 33o.a) Find the refractive index of the plastic.b) If light travels in air at 3.0x108ms-1, find its velocity inthe plastic.

2. Light travels through a diamond at only 1.25x108ms-1.a) Find the refractive index of diamond.b) If a ray of light strikes a diamond surface at an angle of40o from the normal, find the angle of refraction as the rayenters the diamond.

3. Using a laser beam and a fish tank filled with water, therefractive index of the water was found to be 1.33.a) At what incident angle must the beam strike the water toproduce an angle of refraction of 32.5o?b) At what velocity does the laser beam travel in water?

4. Several different angles of incidence and refraction weremeasured for a light ray going from air into a high densitycrystal glass block.a) For each pair of angles, calculate a refractive index value.b) Find the average value for the refractive index.c) Use the average value to find the velocity of light in thecrystal glass.

DATA Angle of Angle ofIncidence Refraction50.0 25.042.0 21.030.0 17.065.0 31.0

continued...

20





Worksheet 4 Part B Practice Problems (continued)

5. Window glass has a refractive index of 1.50.a) Find the velocity of light in this glass.b) If a light ray strikes the glass surface at right angles (i.e. alongthe normal line) what is the value of the angle of incidence?c) Calculate the angle of refraction for this situation.d) How do you interpret this result?

Reciprocal Indices 1n2 = 1 2n1

6. Refer to the information and answers to Q1.a) What is the refractive index for light coming out of theplastic into air?b) If a light ray in the plastic struck the boundary at anangle of incidence of 20o, at what angle of refraction will itenter the air?

7. Refer to Q3.a) What is the refractive index for light travelling from waterinto air?b) If a light ray emerged from water into air at an angle ofrefraction of 37o, what must have been the angle ofincidence?

8. In a type of lead-crystal glass, a light ray exits from theglass into air. At the interface, the angles were i = 15o, andr = 25o.a) What is the refractive index glassnair?b) What is the index airnglass?c) At what velocity does light travel in this glass?

Critical Angle Sin c = gna = 1 ang

9. a) Use the information in Q2 to find the �“critical angle�”for light travelling inside a diamond.b) Describe what would occur (no calculation required) iflight inside a diamond hit the boundary at an angle ofincidence of:

i) 20o ii) 30o

10. a) What is the critical angle for glass with ang = 1.50?b) Describe (no calculation) what happens when light insidethe glass strikes the boundary at angle of incidence:

i) 40o ii) 41.8o iii) 45o

11. Light travelling inside a plastic block strikes theboundary at an angle of incidence = 48.6o. The refractedray is seen to run exactly along the boundary betweenplastic and air.a) What is the critical angle?b) What is the value of anp?c) At what velocity does light travel in this plastic?

FULLY WORKED SOLUTIONS AREIN THE ANSWER SECTION

21







Photo: Anthony Bortolloto

Decorative optical fibres,showing how the light stays inside due to

Total Internal Reflection.

Digital TechnologyIn the past 20-30 years our society has become more andmore �“digitized�”. Because of the speed, storage capacityand processing ability of computers, almost every aspect ofour society has �“gone digital�”.

This simply means that all information (data) whether it bea person�’s voice, written words, numbers, music,photos, etc. is converted into digital code for processing,storage or transmission and communication.

A simple list of some of the technologies involved is:CD�’s & DVD�’s, Mobile phones, digital cameras, theinternet, MP3 music, ATM�’s, computers and theirnetworks.

Increasingly, WAVES are involved in these technologies,especially when data is moved around...COMMUNICATION.

GPS is a system that allows a ship, aircraft, car or even abushwalker, to locate their exact position anywhere onEarth instantly and continuously.

The system was developed for miltary uses, but then madeavailable to anyone. The military version is thought to beaccurate to within a few metres, the civilian version towithin about 20-50 metres.

The system is based on a fleet of satellites positioned inorbit so that from anywhere on Earth, at any moment, atleast 4 satellites are in �“line of sight�”.

Each satellite constantly sends out radio signals identifyingitself, and the exact time the signal was sent.

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

By doing the same for 2 other satellites, the GPS unitrapidly �“triangulates�” the signals from 3 satellites to pin-point your location on the Earth�’s surface. (Aircraft need a4th signal to get their altitude)

Cars can now be fitted with GPS systems that show yourposition on a screen, overlaid onto a road map of the area.As you drive around, the system constantly shows yourchanging position, and can advise you where to turn toreach a designated destination.

22


5. DIGITAL COMMUNICATION & DATA STORAGE




TECHNOLOGY CASE STUDY:

GLOBAL POSITIONING SYSTEM(GPS)

DIGITALTECHNOLOGIES

Satellite orbitsSSaatteelllliittee 11

SSaatteelllliittee 22

SSaatteelllliittee 33

GGPPSS rreecceeiivveerr

Earth

Photo by Pip

Photo by John de Boer

Photo by John de Boer

Computer Hard Drive




23

CONCEPT DIAGRAM (�“Mind Map�”) OF TOPICSome students find that memorizing the OUTLINE of a topic

helps them learn and remember the concepts and important facts.Practise on this blank version.

TTHHEE WWOORRLLDDCCOOMMMMUUNNIICCAATTEESS

Practice QuestionsThese are not intended to be "HSC style" questions, but tochallenge your basic knowledge and understanding of thetopic, and remind you of what you NEED to know at theK.I.S.S. principle level.

When you have confidently mastered this level, it is stronglyrecommended you work on questions from past exampapers.

Part A Multiple Choice

1. A sound wave is best described as:A. mechanical and transverse.B. electromagnetic and transverse.C. mechanical and longitudinal.D. electromagnetic and longitudinal.

2. Which measurement inthis diagram (A,B,C or D)correctly shows the�“amplitude�” of the wave?

3. In a Transverse wave, the particles of the medium:A. vibrate perpendicular to the direction of energy flow..B. move randomly in all directions.C. vibrate parallel to the direction of energy flow..D. move with the energy from one place to another.

4. If the period of a wave is 4 seconds, then its frequencyis:A. 0.25 HzB. 0.4 HzC. 4.0 HzD. 1/16 Hz

5. The period ofthis wave is:A. 0.8sB 1.6sC. 3 mmD. 6 mm

6. If a sound wave has a velocity of 330ms-1, and itsfrequency is 660Hz, then its wavelength must be:A. 990 m B. 2.0m C. 0.5m D. 330m

7. If you heard a sound wave with small amplitude and highfrequency, you would describe it as:A. low volume and low pitch.B. low volume and high pitchC. high volume (loud) and low pitchD. high volume and high pitch.

8. Two pulses are travelling towards each other in a rope.

When they meet at point X, :A. they will interfer destructively and cancel out.B. they will reflect off each other and bounce back.C. constructive interference will increase the amplitude.D. all wave motion will stop at point X.

9. The navigation of a bat in the dark, and the �“depthsounding�” from a boat, both work on the principle of:A. ReflectionB. Refraction.C. ResonanceD. Interference

10. Compared to visible light:A. Infra-red has shorter wavelength and lower frequency.B. Ultra-violet has shorter wavelength and lower frequency.C. X-rays have longer wavelength and lower frequency.D. Microwaves have longer wavelength & lower frequency.

11. The radiation from the Sun least likely to reach theEarth�’s surface is:A. Infra-redB. visible lightC. Ultra-violetD. radio waves.

12. The brightness of a light viewed from 40 metres,compared to viewing from 10 metres would be:A. 1/4 as bright C. 1/16 as brightB. 4 times brighter D. 16 times brighter.

13. An example of REFLECTION being helpful incommunication is:A. Radio waves bouncing off the ionosphere.B. A convex dish antenna collects satellite TV signals.C. Using a concave shaving mirror.D. A convex side mirror on a car sees into the �“blind spot�”.

14. The diagrams show a �“carrier wave�”, and the modulatedwave carrying a signal or message. The method ofmodulation used is:A. AMB. FMC. PulseD. Digital.

15. Which of the following does NOT change when a waveundergoes refraction?A. VelocityB. DirectionC. WavelengthD. Frequency

24





A

C

D

B

disp

lace

men

t (m

m)

3

-3

X

Carrierwave

Modulatedwave

time(s)

1.0

16. If a light ray passed from air into one of the followingsubstances, (each at the same angle of incidence) which onewould show the least amount of refraction?A. Water (refractive index = 1.3)B. Diamond (refractive index = 2.4)C. Glass (refractive index = 1.5)D. Perspex (refractive index = 1.4)

17. Light travels in air at 3.0x108ms-1. If the refractive indexof glass = 1.5, then the velocity of light within the glass is:A. 3.0 x108ms-1.B. 2.0 x108ms-1.C. 4.5 x108ms-1.D. 1.5 x108ms-1.

18. The refractive index of water = 1.33.The �“Critical Angle�” for water would be closest to:A. 38o B. 45o C. 49o D. 53o

19. Long distance communication using laser light andoptical fibres is made practical because of:A. Refraction inside the optical fibre.B. Reflection from the ionosphereC. Total internal reflection in the optical fibre.D. Focusing of the light by a concave mirror.

20. The Global Positioning System (GPS) works on:A. laser beams carried in optical fibres.B. radio signals from local �“cell�” transmitters.C. microwave beams focused by dish antennas.D. radio signals from several satellites.

Longer Response QuestionsMark values given are suggestions only, and are to give youan idea of how detailed an answer is appropriate.

21. (3 marks) List the energy transformations that occur from when youspeak into your mobile phone to when the message isreceived at the local �“cell�” receiver.

22. (4 marks) Differentiate between:a) mechanical and EMR waves.b) transverse and longitudinal waves.

23. (5 marks) A sound wave with frequency 400Hz travels through waterat 1,500 ms-1. Show working:a) calculate the wavelength.b) calculate the wave�’s period.

24. (5 marks) The graph describes awave in the ocean.

a) What is thefrequency of the wave?Explain your answer.

b) Given that the wave travels at 12.5ms-1, find thewavelength. Show your working.

25. (3 marks) Use the �“Principle of Superposition�” to sketch theresultant of the 3 waves shown.

26. (4 marks) With a water wave, a �“crest�” is where water has displacedupwards, and a �“trough�” where it displaced downwards, asthe wave moves through.Explain, in similar terms, what happens to air particles as asound wave passes.

27. (3 marks) Re-arrange these members of the EMR spectrum, placingthem in order from lowest to highest frequency.Radio, infra-red, gamma, light, microwaves, X-ray, ultra-violet.

28. (3 marks) Identify a method for detecting each of these EMR types:(choose a different method for each)a) visible lightb) X-rayc) infra-red

29. (3 marks) A lighthouse is viewed from 10km and its light intensity(brightness) measured to be 0.1 units. How bright would itappear if viewed from 1 km? Explain your answer.

30. (3 marks) Discuss briefly a limitation on the use of EMR forcommunication.

25





disp

lace

men

t

time



ddiisspp

llaaccee

mmeenn

tt ((mm

))

3

-3

10 20

time (s)




31. (3 marks) Complete each diagram to show the expected path of eachreflected light ray P, Q and R.

32. (6 marks) In an experiment, angles of incidence and refraction weremeasured as shown.a) Find the refractive index of the plastic. Show working.b) At what speed does light travel in this plastic? Showworking.

26


PP

RR

air



33. (4 marks) Predict the path of this light ray after it strikes theboundary. Explain your reasoning, and show any working.

34. (3 marks)Outline briefly the underlying principles used in oneapplication of physics related to waves.

airQQ

plastic

3333o

2255o

3300o

plastic.

n = 1.40

keep it simple science tm emmaus catholic college sl...

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