preliminary physics topic 1 the world communicates · pdf filepreliminary physics topic 1 ......

KCiC Physics 1 World Communicatescopyright © 2009 keep it simple sciencewww.keepitsimplescience.com.au

Slide 1

keep it simple scienceKey Concepts in Colour

Preliminary Physics Topic 1The World Communicates

Usage & copying is permitted according to the following

Site Licence ConditionsA school (or other recognised educationalinstitution) may store the disk contents in multiplecomputers (or other data retrieval systems) tofacilitate the following usages of the disk contents:

• School staff may print unlimited copies on paperand/or make unlimited photocopies at one schooland campus only, for use by students enrolled atthat school and campus only, for non-profit,educational use only.

• School staff may use the disk contents to makeaudio-visual displays, such as via computernetworks, or by using data projectors or overheadprojectors, at one school and campus only, forviewing by students enrolled at that school andcampus only, for non-profit, educational use only.

• School staff may allow students enrolled at thatschool and campus only to obtain copies of the diskfiles and store them in each student’s personalcomputer for non-profit, educational use only.

IN SUCH CASE, THE SCHOOL SHOULDMAKE PARTICIPATING STUDENTS AWARE

OF THESE SITE LICENCE CONDITIONSAND ADVISE THEM THAT COPYING OF

DATA FILES BY STUDENTS MAYCONSTITUTE AN ILLEGAL ACT.

• In every usage of the disk files, the KISS logo andcopyright declaration must be included on eachpage, slide or frame.

Please Respect Our Rights Under Copyright Law

®

keep it simple science

KCiC Physics 1 World Communicatescopyright © 2009 keep it simple sciencewww.keepitsimplescience.com.au Slide 2 Usage & copying is permitted according to the

Site Licence Conditions only

®


Preliminary Physics Topic 1

The World CommunicatesFirst, Some Revision: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).

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 EnergyWithout the Transfer of Matter

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

up & down, but goes nowhere.

TYPES of WAVESExamples of energy which movesaround as waves include

SOUND LIGHT

RADIO SIGNALS WATER WAVES

MICROWAVES

... and many more

ENERGY CONVERSIONSEnergy can be converted from one form to another.

In your mobile phone the SOUNDWAVES of your voice are converted toELECTRICAL signals, then transmittedas RADIO WAVES to your friend, whose

phone converts it back again.

SOUND ELECTRICAL RADIO

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

for communication.

X-RAYS


Slide 3 Usage & copying is permitted according to the Site Licence Conditions only

Wave Types &Properties

Nature ofSound.

Speed, Pitchand Loudness

Superposition& Interference

Production,Detection,Dangers

Inverse Square Law

EM Waves inCommunication

Law of Reflection

Light & Mirrors.Reflection in

Communication.

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

Internal Reflection

The EMSpectrum

The WaveEquation

GraphingWaves

The WorldCommunicates

1. Waves2. Sound

Waves

3. ElectromagneticWaves

4. Reflection & Refraction

5. DigitalCommunication

& Storage

®




1. THE NATURE OF WAVESWaves Carry Energy

... or in 2 dimensions:

Ripples spreading onthe surface of a pond.

...or in 3 dimensions,

such as when light radiates in all directionsfrom a glowing object.

Waves & MediumsMechanical waves are those which need a “medium” to travel through.For example, a water wave must have water to travel in. Sound wavesneed air, or water, or some substance to move in. They CANNOT travelin a vacuum.

Electromagnetic (EM) waves do NOT need a medium... they can travelthrough a vacuum, and in fact travel fastest in a vacuum. EM wavesinclude light, radio waves, ultra-violet and other types, and are studied indetail in a later section.

Pulses moving along a slinky spring

CCoommpprreesssseedd sseeccttiioonnss iinn tthhee sspprriinngg mmoovvee aalloonngg iitt lliikkee aa ““MMeexxiiccaannWWaavvee””...... eenneerrggyy iiss ttrraannssffeerrrreedd,, bbuutt tthhee ccooiillss mmeerreellyy oosscciillllaattee bbaacckk

aanndd ffoorrtthh aanndd ddoo nnoott aaccttuuaallllyy ggoo aannyywwhheerree..

®


Waves carry energy,without the transferof matter.

This can occur in 1dimension:

Describing WavesA wave is a vibration. In a mechanical wave, the “particles” (atoms &molecules) in the medium vibrate to transmit the wave energy. In EMwaves 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 theenergy moves across.

If the rope is wiggled constantly up-and-down, you get not just onepulse, but a periodic wave with one pulse following another.

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

TTRROOUUGGHH

EEnneerrggyy mmoovveessaalloonngg tthhee rrooppee

rrooppee

A PERIODIC WAVE

TTRROOUUGGHH

CCRREESSTTEEnneerrggyy mmoovveess

MECHANICAL WAVES require a medium to travel through.

ELECTROMAGNETIC WAVES do not.

A PULSE WAVE is a single wave disturbance.

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



®


Rope vibrates up and down

A PERIODIC, TRANSVERSE WAVE

TTRROOUUGGHH

CCRREESSTTEnergy moves

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

TRANSVERSE WAVESvibrate at right angles to the direction

that the energy is moving.

Energy flow

Vibration in medium

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

EEnneerrggyy mmoovveess

LONGITUDINAL WAVE IN A SPRING

ccoommpprreessssiioonniinn sspprriinngg

rraarreeffaaccttiioonn((wwhheerree sspprriinngg iiss

ssttrreettcchheedd))

SSpprriinngg vviibbrraatteess

Earthquake Shock Waves occur in differentforms, both Transverse & Longitudinal.

LONGITUDINAL WAVESvibrate back-and-forth in the same

direction that the energy is moving.

Energy flow

Vibration in medium



®


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

The Greek letter “lambda” λλ is used as the symbol forwavelength.

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.Frequency is the number of waves that pass a given pointin 1 second, or the number of complete vibrations persecond.

S.I. unit is the “hertz” (Hz) 1 Hz = 1 wave per second.

Wave MeasurementsAll 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 simplerelationship betweenFrequency and Period... theyare 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 = fλλ

WAVELENGTH

AMPLITUDEWave cycles per secondis FREQUENCY

T = 1 and f = 1 f T



®


Example Problem 1A water wave in the ocean has a wavelength of85m, and a velocity of 4.5ms-1.a) Find the frequency. b) What is the period?

Solutiona) V = f λλ

4.5 = f x 85f = 4.5 / 85

= 0.053 Hz (5.3 x 10-2 Hz)(i.e. only a small fraction of a wave passes by eachsecond.)

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 CalculationsExample Problem 2A sound wave has a period of 2.00x10-3s. (T= 0.002s) Sound travels in air at a velocity of330ms-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 λλ330 = 500 x λλ

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


Slide 8

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

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 cork againsttime, the graph will look something like this:

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

Cork bobs up and down

Ripples

00..22 00..66 11..00 11..22

00-33

++33

DDiiss

ppllaacc

eemmeenn

tt ((cc

mm))

OOnnee ppeerriioodd== 00..88 ss

00..44 00..88

TTiimmee ((ss))

®


What you CAN read from a Displacement-Time graph:

Amplitude The vertical scale measures thedisplacement 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 caneasily read from the graph how long it takes for onecomplete 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. λλ = 0.45m)V = f x λλ

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

Graphing a Longitudinal WaveYou 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 “equilibriumposition”, instead of up-down displacement.

Amplitude, Period and Frequency can all be determined in exactly the same way.



LongerWavelength

LowerFrequency

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)

®

keep it simple scienceRelationship Between Wavelength & Frequency

ShorterWavelength

HigherFrequency

To have the same speed, theshorter waves must vibrate at a

higher frequency



Activity 1The 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?

®


Wave P

Wave Q



®


Sound WavesSound waves are Mechanical (they need a medium)

and Longitudinal (vibrate back-and-forth in 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 sound travels. The atomsand molecules are alternately “squashed together” and then stretchedapart as the energy flows through.

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

Velocity of SoundSound 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-1 in water...about 5,000ms-1 in most metals.

FREQUENCY = “PITCH”When you hear sounds of different “pitch” that is the way your braininterprets sound waves of different frequency.

Low Frequency = Low Pitch High Frequency = High Pitch

AMPLITUDE = LOUDNESS or VOLUMESound waves with different amplitudes are interpreted by your brain assounds of different loudness or volume.

Larger Amplitude = Louder Sound Smaller Amplitude = Quieter Sound

2. THE PROPERTIES OF SOUND WAVES

Sound Travels

CCoommpprreessssiioonn CCoommpprreessssiioonnRRaarreeffaaccttiioonn RRaarreeffaaccttiioonn

CCoommpprreessssiioonnss.. HHiigghheerr aaiirr pprreessssuurree

RRaarreeffaaccttiioonn.. LLoowweerr pprreessssuurreeDDiiss

ppllaacc

eemmeenn

tt ffrroo

mmtthh

ee eeqq

uuiilliibb

rriiuumm

TTiimmee

The back-and-forth vibration of the mediumproduces a typical waveshape if graphed.



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 theechoes to help locate their prey, or to navigate, inenvironments where they can’t see very well, such as murkywater (dolphin), or in darkness (bat).

““SSqquueeaakkss”” ooff ssoouunndd

EEcchhooeess ffrroomm iinnsseecctt

SONAR SOund Navigation And Ranging

BAT

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

underwater objects... fish orsubmarines, it all works the

same way.

USES OFSONAR The time delay between sending a

sound “ping’ and receiving the echo,gives depth and distance

AAnnttii-SSuubbmmaarriinneeWWaarrffaarree

DDeepptthhSSoouunnddiinngg

®




®


However, if the waves are “out of phase” (forexample, if compression coincided withrarefaction) then there is destructiveinterference... the opposite amplitudes maycancel each other out.

Theoretically, if 2 sound waves had the sameamplitude and were perfectly “out of phase”they could cancel out totally... imagine having2 sounds that add up to SILENCE! (or 2 lights that combine to form DARKNESS!)

In practice, this only happens over shortdistances or time periods to give “interferencepatterns” and “beat sounds”.

AAdddd ppoossiittiivvee &&nneeggaattiivvee ddiissppllaacceemmeennttss

aatt tthhee cciirrcclleedd ppooiinnttss wave A

wave BDis

plac

emen

t

Resultant

The Principle of SuperpositionAll waves have the ability to pass through other waves withoutbeing affected. For example, you could shine a red spotlightacross a beam of blue light, each colour and beam will emergeon the other side exactly the same.

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

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

In this case, the waves A&B were “in phase” (crest co-incidedwith crest, trough with trough) so the result was constructiveinterference... the resultant has an amplitude which is the sumof A+B.

TToo ffiinndd aa““rreessuullttaanntt””,, aadddd tthheeddiissppllaacceemmeennttss ooffAA&&BB aatt ccoonnvveenniieenntt

ppooiinnttss ((cciirrcclleedd))

DDiiss

ppllaacc

eemmeenn

tt

“resultant” A+B

wave Awave B





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 sketchthe graph of the “resultant” wave. Describe the type of interference in eachcase.

®


DDiiss

ppllaacc

eemmeenn

tt wave Awave B

wave C

wave DDis

plac

emen

t



EM WavesElectromagnetic waves are Transverse waves which do NOT requirea medium to travel through. They travel through a vacuum at3.00x108ms-1, the “speed of light”. They can travel through manyother substances at slightly slower speed. For example, light cantravel 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 asvibrations of electric and magnetic fields, not as vibrating particles.

MEMBERS OF THE EM SPECTRUMRadio (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 atdifferent wavelengths and frequencies.

3. ELECTROMAGNETIC WAVES

Wav

elen

gth

decr

easi

ngve

ry s

hort

v.lo

ng

Freq

uenc

y in

crea

sing

very

hig

hlo

w

Production of EM WavesAll EM waves are produced inbasically the same way: vibration oroscillation of electrically chargedparticles.

For example....Radio waves are produced by electriccurrents running back-and-forth in aconducting wire.

Infra-red waves are made by moleculesvibrating rapidly because of the heatenergy they contain.

Light is emitted when electrons rapidly“jump” down from a higher to a lowerorbit around an atom.

Gamma waves come from thevibrations of charged particles withinan atomic nucleus, during a nuclearreaction in the atom.

®




®


Just as all EM waves are produced in the same basic way, they areall 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 inreal life) the opera singer hits ahigh note and all the wine glassesbegin to vibrate and then shatter...a fictional example of resonance.

Some real examples...

When radio waves hit a suitable aerialwire or antenna, they cause some

electrons in the metal to oscillate back-and-forth “in sympathy” with the wave.

These oscillations are amplifiedelectronically and the signal converted

to sound in the speaker, allowing you tolisten to the radio.

When the fatlady sings...

Antenna

Detection & Reception of EM Waves

When infra-red waves hityour skin they cause certain

molecules to begin toresonate and vibrate. This

sets off nerve messages tothe brain and you feel

warmth or heaton your skin.

In a film camera the light causes resonance inchemicals in the film. Chemical reactions occurwhich permanently alter the film so that animage appears when “developed” later.

Different film can besensitive to infra-red,(photos in the dark)or X-rays for medicaluses.

All waves are detected when they causeresonance vibrations.



®


A little UV gives you a suntan, but long-term exposureleads to skin damage, premature skin “ageing”, and isa major cause of deadly melanoma skin cancer.

The Sun produces dangerous quantities of UVradiation, 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

ssoommeerreefflleecctteedd

rraaddiioo

iinnffrraarreedd && lliigghhtt

Danger of High Frequency EM WavesHigh 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 permolecule (O3) instead of the normal 2 (O2).

The ozone moleculesresonate well at the frequency of UV and so absorb it strongly.

The Sun only produces smallamounts of the even more dangerousX-rays and gamma radiation. Onceagain, most is absorbed in the upperatmosphere, this time by ordinary oxygen and nitrogengases.

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

UV RaysOxygen O22

does notAbsorb UV

Ozone O33Absorbs

UV



®

keep it simple scienceThe Inverse Square Law

As any form of radiation spreads out from its source its intensity gets less. For example, a sound becomes quieter if you’re further from the source,

or a light is not so bright as you move further from it.At distance “d” from the light source, some lightenergy falls on an area of x2 units. At twice thatdistance (2d) the same amount of light would fallon an area of 4x2. The brightness of the light mustbe only 1/4 as much (since the same amount oflight 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 inproportion to the distance squared, in each case.

Mathematically, the relationship is that the intensity (I) (suchas brightness of light) is inversely proportional to theSQUARE 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

lliigghhttssoouurrccee

ddiissttaannccee ““dd””

ddiissttaancee “22dd”

22xxSSqquuaarree

AArreeaa xx2

SSqquuaarree wwiitthhssiiddeess ttwwiiccee aasslloonngg..

AArreeaa == 44xx2

SSaammee aammoouunnttooff lliigghhtt ffaallllssoonn 44 ttiimmeesstthhee aarreeaa





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 wasfound to be 16 units. What intensity would be measured at a distance of:a) 4 metres?b) 8 metres?c) 1 metre?

®



Slide 20


®


Radio & Microwaves carry radioand TV broadcasts, telephone long-distance links, mobile phonenetworks, and satellite links fortelephone (including internet) and TV.

If you have “Satellite TV”, the “dish”on your roof is an antenna to receivemicrowaves directly from an orbitingsatellite.

Add to that, 2-way radio for militaryuses, CB amateurs and boating,shipping and aircraft communications,and you begin to realise how manyradio 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 theyinterfere constructively to form a veryintense, tight beam.

•A laser beam will stay inside an opticalfibre and not “leak” out or dissipate forlong distances.

•A laser can be turned on & off veryrapidly, so it’s perfect for high speeddigital communication.

EM Waves & CommunicationHumans 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 formof LASER beams carried in opticalfibres for telephone and internetcommunication.

KCiC Physics 1 World Communicatescopyright © 2009 keep it simple sciencewww.keepitsimplescience.com.au Slide 21 Usage & copying is permitted according to the

Site Licence Conditions only

®


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 3 common ways to modulate the wave to carry information...

Frequency Modulation (FM)The amplitude stays constant while the

frequency (and wavelength) varywithin a fixed range. The information(voice, music etc) is “coded” in the

variations of frequency.

FM radio gives much better fidelity andis 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 theinformation being carried... whether

voice or music, or whatever.

DDiiggiittaallssiiggnnaall

DDiiggiittaall 11 00 11 11 00 11ddaattaa

WWaavvee ppuullsseessoonn aanndd ooffff

This diagram compares the effect of AM, FM & Digital Modulation

on the same “carrier wave”

WAVEMODULATION

Pulse Modulation(Digital)

To carry information in digital form thewave must switch rapidly between 2

different states, representing the “1” and“0” of digital codes. The wave can beswitched rapidly on and off (as in thediagram) or switched back-and-forthbetween different “phase states”...

phase modulation.

OpticalFibres carry PulseModulatedlaser beams

““CCaarrrriieerrwwaavvee””

NNooiinnffoorrmmaattiioonnccaarrrriieedd

AAMMssiiggnnaall

AAmmpplliittuuddeecchhaannggeess..FFrreeqquueennccyyccoonnssttaanntt

FFMMssiiggnnaall

FFrreeqq.. cchhaannggeess..AAmmpp..ccoonnssttaanntt



®


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

1. The SOUND energy ofyour voice is convertedto ELECTRICAL signalsby the microphone. Theelectrical signal is usedto digitally modulate a

RADIO wave.

2. The digital RADIOsignal is

transmitted byyour phone

and receivedby the local

“cell” antenna.

3. If your call is going to aperson in another location (adifferent “cell”) the signal isconverted into a modulated

MICROWAVE and beamed, viahilltop relay towers, to the

correct area. (Alternatively, it might be sentas a modulated Laser LIGHTbeam through optical fibres).

4. In the other cellarea, the signal is

converted back to amodulated RADIO

signal andtransmitted.

SOUND ELECTRICITY RADIO MICROWAVE RADIO SOUND(or LASER LIGHT)

ENERGY CHANGES

5. Your friend’sphone receives

the RADIO signal,amplifies it as an

ELECTRICALsignal and this is

converted toSOUND waves intheir earphone.



®


In the future we will need to switchmore communications to use thelaser light & optical fibre methodwherever possible, and to makebetter use of the RF bands. Forexample, it is possible to use thesame frequency “channel” forseveral different purposes as long asthe different signals are modulateddifferently and as long as the radioreceivers are sophisticated enoughto pick out only the desired signaland ignore the others.

One thing is for sure... humans willkeep communicating and the needfor new services will keepexpanding. So far, our technologyhas always managed to keep up, andit will probably continue to do so.

Modern communication systems have developedrapidly and new features and capabilities seem tocome out every day. It seems that the entiresystem is unlimited and that it can continue toexpand and improve forever.

Well perhaps it can, but NOT while continuing touse the radio end of the EMR spectrum. Each“station” or channel must operate on a differentfrequency or else signals can “jam” or “interfere”with each other.

The simple fact is that there are now so manyradio & TV stations, mobile phone networks,aircraft and shipping channels, military, policeand emergency service channels, etc. etc. allusing the RF (Radio Frequency) part of the EMRspectrum, that it is becoming difficult to keepexpanding services without interfering withexisting channels.

Discussion:LIMITATIONS OF COMMUNICATION CHANNELS





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 changesoccurring during a mobile phone call.

3. Generally, the higher the frequency of a wave the more data and informationcan be carried.

a) Relate this fact to the increasing use of microwaves and lasers incommunication.

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-lightlasers. Why?

®



Slide 25


®


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.

4. REFLECTION & REFRACTION

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

AAbbssoorrbbeedd eenneerrggyybbeeccoommeess hheeaatt

ABSORPTION ofthe energy

REFLECTION(bounces off)

TRANSMISSION into thenew medium, with possible

REFRACTION(change of direction)

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 is still obeyed, as shown:

“Normal”line

IInncciiddeenntt rraayy

RReefflleeccttee

dd

rraayy

ioo

roo

Reflectivesurface

such as amirror

The Incident rays P,Q & R are parallel.

Each obeys the Law of Reflection, but thereflected rays go in different directions.

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

P QR

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



®


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

Concave mirrors can give ENLARGED images ifviewed from the right distance, such as ahousehold shaving mirror or make-up mirror,which gives a magnified reflection of your face.This is also the basis of a reflecting telescopeswhich is the main type used in Astronomy.

Focus

Reflection of Light from Curved Mirrors“Convex” mirrors reflect light so the raysdiverge outwards, as if coming from a focusbehind the mirror.

Convex mirrors produce smaller (“diminished”)images, but give a wider-angle view. An exampleof use is the side mirrors on a car which giveyou a wide-angle view into the driver’s “blind-spot”. (BUT things look smaller. This canconfuse a driver into thinking that other cars arefurther away.)

“Virtual”Focus



®


Wave reflection from the ionosphere can help withlong distance radio communications. It worksbest with the longer wavelength AM signals.

The Ionosphere is a zone in the upper atmospherewhere the air molecules are partly ionised(electrically charged) by radiations from the Sun.The ionised gases act as a reflective surface toradio waves of certain wavelengths.

TV signals and FM (shorter wavelengths) radio donot reflect so well and generally you need to be in“line of sight” from the transmitter to get goodreception.

Transmitter

Ionnosspphheerreellaayyeerr

Receiver

EARTH

Another example involves howMicrowaves are transmitted andreceived. Microwaves are used torelay TV programs to regionaltransmitters and to relay longdistance phone calls (includinginternet) from city to city.

At the transmission end, a curvedreflector keeps the waves in a tightbeam aimed at the next relaystation. The receiver has a similardish to focus the waves into the receiving antenna.

MMiiccrroowwaavveebbeeaamm ttrraavveellssbbeettwweeeenn rreellaayy

ssttaattiioonnss

YYoouurr ssaatteelllliittee TTVV ddiisshh iiss aa rreefflleeccttoorr ttoooo

Microwave Reflector Dishes

TTrraannssmmiitttteerrddiisshh

RReecceeiivveerrddiisshh

Reflections in Communications



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

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

In this case,

io < ro

ioro

Incident Ray Glass Air

Refracted Ray

nnoorrmmaall

RefractionRefraction 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.

io > roio

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 ofIncidence

Angle ofRefraction

®



Slide 29


®


You may have carried out an investigation in classusing a “Ray Box Kit” to measure angles ofincidence and angles of refraction of light rayspassing into a glass block.

When you graph the angles the result is a curve.

This is not much use for defining any relationshipthat may exist.

Angle of refraction, roo

Angl

e of

inc

iden

ce, i

oo

Snell’s Law In 1621, Willebrord Snell discovered that if yougraph the Sine ratios of the angles, the points lie ina straight line. You may have done the same withyour experimental data.

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

The gradient of the line is not only the ratio between the Sine of the angles, but is alsoequal to the ratio of velocities of the wave in the 2mediums involved.

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

This is now called Snell’s Law:

Sin roo

Sin

ioo

GGrraaddiieenn

tt == rriiss

ee == SSiinn

ii

rruunn SSiinn

rr

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

Sin i = V1 = 1n2Sin r V2



®


When waves enter a new medium, and then exit it again, therefractions that occur on the way in, are the opposite of whathappens 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 = 1.5

and

Refractive Index(glass -> air) gna = sin28 / sin45 = 0.66

These 2 values are RECIPROCALS !! ...and this will alwaysbe the case... the index of refraction going in is the reciprocalof the index coming out.

1n2 = 1 2n1

The spoonappears “broken”at the surface ofthe tea due to

refraction of thelight by which

we see it.

45oo

45oo

28oo

28ooRefractionair ->> glass

Refractionglass ->> air

nnoorrmmaall

glass

3 beams of light being refracted through a perspex block.

Refractive Index



®


Snell’s Law Sin i = V1 = 1n2Sin r V2

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

Solution a) Sin i = n sin 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 = 1.50V2

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

Snell’s Law Calculations



®


This means that the Sine Ratio of the critical angle “C” isequal 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 ofthe glass, but does not cross the boundary.

So, when the angle of incidence equals the “criticalangle”, the angle of refraction is a right angle.

If io = co, then ro= 90o

Remember that Sin i = gna Sin r

so at the critical angle Sin c = gna

Sin 90

and sin 90o = 1, so...

Consider the situation when waves are going froma 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 theincident angle as shown in this

series of diagrams. ioo

roo

1

ggllaassss

aaiirr

iooroo

2

bigger i,bigger r

ioo=cooroo= 90oo

3

CCrriittiiccaall AAnnggllee

Sin c = gna = 1 1 ang



®


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

“TOTAL INTERNAL REFLECTION”

This has one very important application incommunication technology...

Optical fibres are thin strands of very pure glassthat can carry communications signals in the formof laser light beams. The laser beams stay withinthe fibres because of total internal reflection.

If ioo > coo

the ray cannot get out.It reflects back inside

the glass.ioo>coo

Rayreflectsinsideglass

4

What Happens Beyond the Critical Angle?

This diagramfollows onfrom theprevious

slide.

Each fibre is a core strand of glass, with anotherlayer wrapped around it. The outer layer has alower refractive index than the core, so even wherethe fibre bends around a corner, the laser light willgenerally strike the boundary at an incident anglegreater than the critical angle.

Whenever the laser beam hits the boundarybetween the 2 layers, the angle of incidenceexceeds the critical angle, (io > co) so Total InternalReflection occurs and the beam stays totally withinthe fibres over long distances.

The laser light “bounces” aroundcorners by total internal reflection

Optical fibrellaasseerr bbeeaamm

Lower indexouter layer.

Core.High index.





REFLECTION & REFRACTION Student Name .................................

1. List the 3 possible things that can happen when a wave strikes a boundarybetween 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 adenser medium such as water or glass.

4. (fill in the blank spaces) “Refractive index” can be measured as the ratiobetween ..................................... of the angles of ............................. and.........................., OR as the ratio between the .............................. of light in eachmedium. The index for light entering a medium is the ............................... of theindex 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?

®




Digital TechnologyIn the past 20-30 years our society has become more andmore “digitised”. 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 be a person’s voice,written words, numbers, music, photos,etc. is converted into digital code forprocessing, storage or transmission andcommunication.

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’sGPS

Increasingly, WAVES are involved in thesetechnologies, especially when data is movedaround... COMMUNICATION.



®


GPS is a system that allows a ship, aircraft, car or 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 metre, the civilian version to withinabout 10 m.

The system is based on a fleet of 32 satellites (controlledby the US Air Force) positioned in orbit so that fromanywhere onEarth, at anymoment, severalsatellites are in“line of sight”.

Each satelliteconstantly sendsout microwavesignals identifyingitself, its orbitdetails and theprecise time thesignal was sent.

TECHNOLOGY CASE STUDY: GLOBAL POSITIONING SYSTEM (GPS)

GPS

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)

GPS systems for cars show your position on a screenoverlaid onto a road map of the area. As you drive around,the system constantly shows your changing position, andcan advise you where to turn to reach your destination.

When yourportable GPSreceiver picks upthe signal, it cancalculate yourexact distancefrom the satellite,from the timedelay since thesignal was sent.

Satellite orbitsSatellite 1

SSaatteelllliittee 22

Satellite 3

GGPPSS rreecceeiivveerr

Earth

preliminary physics topic 1 the world communicates · pdf filepreliminary physics topic 1 ......

Documents