p82n World Communicates

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The World Communicates

Syllabus 8.2.1

[Oct 2002 Revision of Physics Syllabus]

2006 Edition Physics Topic 8.2 page 1


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Waves as Carriers of EnergyAll waves carry energy. Evidence for the energy that wavespossess can be seen in

microwaves that cook food

x-rays that can damage DNA molecules in living cells

earthquake waves that can knock down buildings

ultrasound waves that can warm human flesh

sound waves that can make small objects move

water waves that can move even massive ships

Light and EnergyLight is a form of energy. Light energy, captured and storedby plants during the process of photosynthesis is essential tomost living things. The energy of light may be converted toelectrical energy by a solar cell, a semiconductor device,usually made of silicon. Light energy also produces chemicalchanges on a photographic film which make photographypossible. In a closely related process, if light is allowed to fallon the chemical silver chloride which is related in propertiesto the chemicals used in photography, the energy of the light converts the silver chloride to silver and causeschlorine gas to be produced. Video and digital cameras use the energy of light to activate special semiconductormaterials (called a charge coupled device, CCD) to produce electrical signals upon which the electronic imageultimately depends.

Waves Carry Energy Away from a Vibrating SourceWave motion is the result of a periodic disturbance of a medium, or of space by some form of vibration (oroscillation ), which transmits energy away from the oscillating source of the wave.

Some examples of waves and their sources of energy are summarised in the following table.

Type of Wave OriginWater waves Any movement on the surface of water creates surface water waves a moving

animal, wind, dropping an object into the water. Undersea earthquakes produce asurface wave called a tsumami. sunamis can be very destructive when they reachland because of their great energy.

!ound waves !ound waves are created by vibration of an object in, or in contact with, a mediumthrough which the sound can travel. e.g. the human vocal cords, the cone of aloudspeaker, vibrating strings and reeds in musical instruments, and all sorts ofmovements and vibrations in the environment. "ur ability to sense sound is a key tocommunication and survival.

#arthquake waves !udden movements of the earth$s crust, at geological faults for e%ample, result in arelease of energy, which causes vibrations that may spread over and through theentire earth.

#lectromagnetic waves All electromagnetic waves &e.g. light, radio waves, microwaves and infrared waves'are produced by the vibration of charged particles.

A wave propagates (travels) away from the vibrating source of energy. Waves can propagate as one- , two- orthree-dimensional disturbances.

One-dimensional In the case of a one-dimensional wave, the energy travels effectively in a straight lineaway from the source of the wave. e.g. sound confined to a tube such as a flute,digeridoo or organ pipe; a vibration travelling along a string/spring, a laser beam(effectively one-dimensional)

Two-dimensional The energy associated with a two-dimensional wave spreads out in a plane or flatsurface. e.g. surface water waves, the vibrating skin on a drum, surface earthquakewaves.



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Three-dimensional In the case of a three-dimensional wave, the energy spreads out into the spacesurrounding the source in all directions. e.g light from a candle or light bulb, sound inair, radio waves from a radio stations transmitter, microwaves from a mobile phone.

Waves can be categorised as one of two types, the distinguishing feature between the two types being whether amedium is required for the energy associated with the wave motion to propagate . The two types are

Mechanical Waves . These are waves requiring a medium through which to propagate. The particles of whichvibrate when the wave travels, are sometimes referred to as mechanical waves . These include sound waves ,water waves , earthquake (seismic) waves , and waves which can travel as a vibrating disturbance throughelastic materials such as stretched strings or springs , membranes or any other form of matter.

Electromagnetic Waves. Waves not requiring a medium to travel through are called electromagnetic waves .Electromagnetic waves cover a continuous range from gamma rays , through a variety of others (see figurebelow) including light , to radio waves . Electromagnetic waves, unlike all other types of waves, can travelthrough a vacuum. Electromagnetic waves may can travel through matter as well, but they interact with thematter, an may be absorbed or reflected as well important properties of electromagnetic waves.

3

Figure below: The components of the electromagnetic spectrum.

3All waves have in common

a source of energy, which involves vibration of some sort the rate of the vibration, measured in hertz(Hz) (one hertz is one cycle or vibration per second) is called the frequency

a means by which the energy can propagate outwards away from the source as a vibration the rate atwhich the wave travels away from the source is called the speed or velocity of the wave and is measuredin metres per second (ms 1) in the SI system of units

the transformation of energy from one form to another

a transfer of energy from one place to another

Waves can be also be categorised as either of two main types 1

transverse

longitudinal

3These are discussed in detail in following sections.

1 There are other types, such as torsion waves, however they are not included in this course.



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Wave fronts in #$dimensiona Waves

Longitudinal WavesA longitudinal wave can be produced in a slinky by stretching the spring, so that it is straight but not over-stretched, and then pushing one end suddenly towards the other, and then reversing this action.

This produces a region of compression , where the coils are pushed closer together. These in turn push on theadjacent coils, compressing them. Meanwhile, returning the displaced end to its original position now produces aregion on the slinky over which the coils are stretched apart further than when they are at equilibrium. Thisregion is called a rarefaction . The compression and rarefaction travel along the spring as a wave disturbance.

Pushing and pulling the end of the slinky repeatedly sets up a periodic wave called a longitudinal waveconsisting of alternating regions of compression and rarefaction. Like a transverse wave, the propagation of alongitudinal wave requires that there be a medium to vibrate. Like a transverse wave, energy be transferred throughthe medium as the longitudinal wave propagates away from the source of the wave.

In contrast to the transverse wave, the motion of the particles in the medium through which the longitudinal wave istravelling is parallel to the direction of energy propagation.

Like a transverse wave, a longitudinal wave has the properties of velocity , wavelength , frequency and amplitude .

The wavelength of a longitudinal wave is equal to the distance between the centres of successive compressions orsuccessive rarefactions (or any other two successive corresponding points on the wave).

The amplitude of a longitudinal wave is difficult to represent on a stationary diagram. It is the distance any oneparticle moves from its equilibrium position to its extreme distance from that position. It is much less than the



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wavelength of the longitudinal wave. In this case, it is the maximum distance the end of the spring is pushed fromits equilibrium position.

The velocity of the wave, as with transverse waves, is the speed at which the energy propagates through the springor other medium in which the wave is travelling.

The frequency is the number of oscillations per second the particles in the medium undergo. As with any

frequency measurement, it is measured in hertz (Hz).The same wave equation relates these quantities.

v = f The period is the time it takes for one complete wave to pass a given point. e.g. the time between the arrival at agiven point of two successive compressions. Period and frequency are inversely related.

T = 1

Investigations

Compare the velocity of a transverse wave with that of a longitudinal wave in a slinky .Use a data logger with microphone sensor to measure the frequency of a vibrating guitar string. Measure the stringlength and hence determine a relationship between frequency and wavelength. (See appendix 1 )

Use a long (2 cm diameter) spring to investigate the frequency/wavelength relationship (using overtones)

Use a flute with CRO or data logger to measure the frequency of the fundamental (C) and successive overtones,produced by a competent flutist over-blowing, to investigate the frequency/wavelength relationship using soundwaves.

The idealised (simplified) harmonics produced by a flute are shown in the following diagram. Musicians shouldnote the successive wave number ratios (2:1 octave, 3:2 fifth, 4:3, 5:4 etc) produced in this sequence.



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Properties of Waves

Definitions, Symbols and UnitsFrequency (f or n): This is the number of waves passing a given point per second. The fre uency

can be measured in waves per second. !t is more usual to call this "hert#".i.e. if $% waves pass a given point each second then the fre uency is $% hert#()

Wavelength (l): The distance between the two successive points on a wave that are inphase with each other. This may be the distance between two ad'acentcrests of the wave or between two ad'acent troughs. The S! units forwavelength are metres (m), but it is common in optics to use nanometres ($nm $% * m) or angstrom units ($ + $% $% m)

Amplitude ( ): The amplitude of a wave is the ma-imum distance a particle in the wavemoves from its normal, undisturbed rest position (also called the equilibriumposition ).The greater the amplitude of a wave the greater its amount of energy.lectromagnetic waves are an important e-ception to this rule. The energy of an electromagnetic wave is determined by its fre uency the greater thefre uency, the greater the energy of an electromagnetic wave.

Period (T): This is the time ta/en for a particle in a wave to go through one completevibration. 0eriod, being time, is measured in seconds. 1otice that period andfre uency are inversely related, that is, T $2f.

Velocity (v): This is the speed at which the energy of the wave is travelling ( propagating )through the medium.

Sound waves travel at about 34% m s 5$. 6ight travels at 3 - $% 7 m s 5$. Thevelocity, fre uency and wavelength of all waves are related by a simplerelationship.

velocity = frequency x wavelength

v = fl

Ray : line drawn in the direction of propagation of the wave is called a ray. 8aysare very useful in describing the behaviour of light waves.

Wavefront : !s the leading edge of a wave of which is characterised by all the particlesvibrating in the same phase. 0hase refers to the fraction of a period relative toa fi-ed reference. The propagation of a wavefront is useful in describing thebehaviour of waves, including reflection and refraction.

isplacement : The distance a wave travels in a medium in a straight line from a referencepoint 98 the distance a particle in a wave moves from a reference point as itvibrates (the ma-imum displacement of a particle from its e uilibrium positionis called the amplitude).

!rest"#rough : Usually refers to the highest points of a transverse wave above thee uilibrium position. trough is the lowest point below the e uilibriumposition.

#ransverse wave : wave in which the vibrations that transmit the energy occur in a directionperpendicular to that in which the energy is propagating.

$ongitudinal wave : wave in which the vibrations that transmit the energy occur in a directionparallel to that in which the energy is propagating.

!ompression : The region of a longitudinal wave in which the particles are closer togetherthan the mean e uilibrium distance.

Rarefaction : The region of a longitudinal wave in which the particles are further apart thanthe mean e uilibrium distance.



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%ow Te ephones Wor!A good explanation of how a telephone works can be read on the excellent web site

http://www.howstuffworks.com/telephone.htm

by Marshall Brain

The Howstuffworks website makes interesting general reading and is relevant to many areas of the physicscourse.

Energy Transformations in a &obi e Te ephoneIn the telephone microphone, sound energy is converted to electrical energy. Electrical energy isused to carry the information through copper wires inside the phone.

A radio transmitter in the mobile phone sends the digitally encoded speech using microwaves.Microwaves are a form of electromagnetic wave.

The electromagnetic wave travels to the receiving antenna on a mobile phone base station, whereit is converted back into electrical energy. The base station eventually uses electrical energy toconvert the signal into a microwave again, which is transmitted to the receiving phone.

In the receiving phone, the microwave energy is transformed into electrical energy once again in

the phones antenna. The phones speaker converts electrical energy to sound energy.The energy to operate the mobile phone is supplied by a battery. A battery stores energy as

chemical energy, which is a form of potential energy. While the phone is switched on, the chemical energy istransformed to electrical energy to operate the phone.

Phones that have illuminated displays, including colour LCD displays, convert electrical energy to light energy tooperate the display.

In some parts of a telephone network, the information is encoded for transmission through an optical fibre. In thiscase, electrical energy is being converted to light energy. At the other end of the fibre, the light energy is againconverted back into electrical energy.

Summary

'art of phone Transforms energy from Transforms energy to

Speaker Electrical energy Sound energy

Microphone Sound energy Electrical energy

Battery Chemical energy Electrical energy

Antenna (transmitting) Electrical energy Electromagnetic energy

Antenna (receiving) Electromagnetic energy Electrical energy

Screen Electrical energy Light energy

%ow a Ce 'hone Wor!s

by Marshall Brainmodified from the excellent website http://www.howstuffworks.com/cell-phone.htm

Each day something like 30 000 people in the United States sign up for and start using a cellular phone (2000figure). Therefore it is likely that you or someone you know has a cell phone and uses it on a regular basis. Theyare such great gadgets - with a cell phone you can talk to anyone on the planet from just about anywhere (80% of the U.S. has coverage, but the Australian digital network is not compatible with the US system and so mobilephones designed specifically for use in Australia will not work in the USA. Mobile phones that work in bothcountries are called tri-band phones, the name referring to the fact that they can send signals at three differentmicrowave frequencies. Non-tri-band Australian digital mobile phones will work in Europe and Asia for a price).But have you ever wondered how a cell phone works? In this edition of How Stuff Works the technology behindcell phones outlined, so that you can see how amazing they really are.

One of the most interesting things about a cell phone is that it is really a radio - an extremely sophisticated radio,

but a radio nonetheless. A good way to understand the sophistication of a cell phone is to compare it to a CB radioor a walkie-talkie. A CB radio is a simplex device. That is, two people communicating on a CB radio use the samefrequency, so only one person can talk at a time. A cell phone is a duplex device, so it uses one frequency fortalking and a second, separate frequency for listening. A CB radio has 40 channels. A cell phone can communicate

http://www.howstuffworks.com/telephone.htmhttp://www.howstuffworks.com/cell-phone.htmhttp://www.howstuffworks.com/telephone.htmhttp://www.howstuffworks.com/cell-phone.htm


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The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low powermeans small batteries, and this is what has made hand-held cellular phones possible.

The transmissions of a base station and the phones within its cell do not make it very far outside the cell.Therefore, in the figure above both of the green cells can use the same 59 frequencies. The same frequenciescan be reused extensively across the city.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can havehundreds of towers.

But because so many people are using cell phones, costs remain fairly low per user. Each carrier in each city alsoruns one central office called the MTSO (Mobile Telephone Switching Office). This office handles all of the phoneconnections to the normal land-based phone system and controls all of the base stations in the region.

So lets say you have a cell phone, it is turned on, and someone tries to call you. The MTSO gets the call, and ittries to find you. In early (pre-roaming) systems the MTSO found you by paging your phone (using one of thecontrol channels, to which your phone is always listening) in each cell of the region until your phone responded. Itthen told both your phone and the base station in the cell, which of the 59 channels in your cell your phone, shouldbe using. At that point you were connected to the base station and you could start talking and listening.

As you move toward the edge of your cell, your cells base station will note that your signal strength isdiminishing. Meantime, the base station in the cell you are moving toward (which is listening and measuring signalstrength on all frequencies, not just its 1/7 th) will be able to see your phones signal strength increasing. The twobase stations coordinate themselves through the MTSO, and at some point your phone gets a signal on a controlchannel telling it to change frequencies. This handoff switches your phone to the new cell.

Roaming makes things a bit more interesting. In modern systems the phones listen for a System ID (SID) on thecontrol channel at power-up. If the SID on the control channel does not match the SID programmed into the phone,then the phone knows it is roaming. The phone also transmits a registration request and the network keeps track of your phones location in a database (this way the MTSO knows which cell you are in when it wants to ring yourphone). As you move between cells, the phone detects changes in the control channels strength and re-registersitself with the new cell when it changes channels. If the phone cannot find any control channels to listen to it knowsit is out of range and displays a no service message.

The latest trend is digital cellular phones. They use the same radio technology (in different frequency bands forexample, PCS phones use frequencies between 1.85 and 1.99 gigahertz) but compress your voice into digital 1s and0s. This compression allows between 3 and 10 cell phone calls to occupy the space of a single analog voice call.

PCS digital phones also offer other features like paging and email.The next time you pick up and use a cell phone especially one of the new tiny ones that fit into your shirt pocket keep in mind all of the technology packed into that amazing little device!



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'hysics Sy abus ()#)#

[Physics Syllabus Oct 2002 Revision]



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Sound Waves Longitudinal WavesSound waves typically originate from vibrating objects such as the human vocal cords, strings on musicalinstruments, loudspeakers, air columns in musical instruments such as clarinets and trumpets, and stretchedmembranes such as those on drums. Sound plays a key role in human communication and entertainment.

Sound waves require a medium through which to travel. Gases, liquids and solids can all transmit sound energy.

Sound waves are longitudinal waves . Particles in the medium transmitting the sound vibrate in a line parallel to thedirection of propagation of the sound. Longitudinal wave motion is characterised by vibration of the mediumparallel to the direction of propagation of the wave.

Sound waves consist of alternating compressions and rarefactions propagating away from the vibrating source.

Investigation

Use a tuning fork to investigate some characteristics of sound including frequency, pitch, loudness, energy (putthe tips of the vibrating tuning fork into a cup of water use plastic, the energy of the vibrating fork can break a beaker)

Use a signal generator connected to an amplifier/speaker to demonstrate sound properties including energy,loudness, frequency, pitch

Examine the production of sound by a variety of musical instruments

Consider the production of a sound wave by the vibration of a tuning fork as shown in the following diagram.

Sound waves propagate as longitudinal waves. In the above diagram, the prongs of the tuning fork vibrate back andforth as a result of energy being applied to them when the fork is struck. As the prong on the left moves to the left,it pushes the adjacent air molecules closer together. This region is called a compression (C). It is a region of higherpressure than the ambient air pressure. The compression travels away from the tuning fork prong as the energy istransferred from one molecule to the next. When the tuning fork prong moves to the right, the air molecules cannotinstantly fill the space and a region of lower pressure results. This is called a rarefaction (R). As the prongcontinues to vibrate, successive alternating compressions and rarefactions are created. The sound wave is shownpropagating to the left. The right prong will produce a wave propagating to the right. This diagram shows the soundwave as being one-dimensional. This is done for simplicity. Sound waves propagate in three dimensions from asource. The principle of the production of the sound is the same as described above. Let us examine the process of sound production in more detail.

In the following diagrams, the dots represent particles making up a substance capable of transmitting a longitudinalwave, in this case it could be a gas in a tube with an open end on the right. In the diagrams, a source of sound, say atuning fork prong is shown on the left side. Diagrams A-K represent the position of the gas particles in the tube atsuccessive intervals of time.

Reference: http://www.physicsclassroom.com/mmedia/waves/gsl.html

http://www.physicsclassroom.com/mmedia/waves/gsl.htmlhttp://www.physicsclassroom.com/mmedia/waves/gsl.html


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In diagram A, the tuning fork is shown just as it begins to push the air molecules to the right, causing them to bunchup against the tuning fork creating a region called a compression . In diagram B, the tuning fork prong is shown atits greatest displacement to the right. As it vibrates, it moves to the left, to the position shown in diagram C. Themolecules which were pushed close together by the fork as shown in diagram B, then push more particles further tothe right closer together. Thus the region of compression continues to propagate to the right.

As the tuning fork prong moves to the left, a region is created in which particles are further apart than average. Thisregion is called a rarefaction . In diagrams D, E and F, a sequence of positions of the tuning fork moving to theright a second time is shown creating a second region of compression.

In the diagrams G and H the movement of the tuning fork prong is to the left, creating a second rarefaction. As thevibration of the source continues, a series of alternating compressions and rarefactions is generated, propagating tothe right.

Sound waves thus propagate through a medium as a series of alternating compressions and rarefactions. This typeof wave is sometimes called a compression wave.

When a tuning fork vibrates, its energy is transferred to the surrounding medium. The sound spreads out in a three-dimensional spherical pattern. In the following diagram, the circular wave fronts are drawn so that they correspond

to the compressions of the wave. The direction of propagation of the wave is radially away from the source.

In practice, the energy does not spread evenly in all directions. This is easy to hear if a tuning fork is rotated aroundits long axis while it is sounding. Variations in the loudness of the sound will be heard these are the result of the

non-uniform pattern of energy radiation. Relatively more energy is transmitted along the line indicated by the wavecompressions CCC, and in the opposite direction. This occurs because of the vibration of the tuning fork back andforth along this line.



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It is difficult to draw a model of a longitudinal wave, especially one showing the amplitude of vibration of theparticles. Longitudinal and transverse waves have in common the periodic oscillation of the particles in themedium. The longitudinal motion of any one particle can be represented on a displacement-time graph as follows.

It should be noted that the sinusoidal graph has peaks (high pressure) corresponding to the compressions andtroughs (low pressure) corresponding to rarefactions. This graph represents the pressure variations at an instant of time, at any point along a line of propagation of the longitudinal sound wave.

As the prong moves to the right, it creates a region of higher pressure called a compression . As the prong moves tothe left, a low-pressure area called a rarefaction is created. A sound thus consists of a series of such alternatingpressure variations travelling away from the vibrating source.

It is thus a common convention to represent longitudinal waves with a transverse model. The following graphshows the position any single particle , as a function of time.

The preceding graph should not be confused with the diagram of a transverse wave shown as displacement fromequilibrium plotted against distance along the line of propagation (as previously discussed in relation to transversewave motion.



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The Speed of SoundThe speed of sound is dependent on the medium through which it travels. It is slowest in gases and fastest in solids.Examples tabulated below show the speed of longitudinal waves (e.g. sound), vL, and transverse waves, vT, invarious media. Liquids do not transmit transverse waves through the body of the liquid. Only surface waves exist inliquids.

For any given material, if it can support the propagation of both types of waves, the longitudinal wave velocity isgreater than the transverse wave velocity in the medium.

MATERIAL Density (gcm 1) VL (m/s)

copper 8.90 6420

steel 7.86 (iron) 5940

beryllium 1.93 12890

aluminium 2.58 6420

water 1.00 1496

ethanol 0.79 1207

air 0.00139 331.45

helium 0.000178 965

fat 0.95 1450

muscle 1.07 1580

skull bone 1.91 4080

A thinking exercise!

Consider this hypothesis: The speed of sound in different materials is greater in materials having a greaterdensity.

Assess this hypothesis, using the information in the table.

Sound velocity in gases is temperature dependent. In air the velocity of sound increases at the rate of 0.59 ms -1C-1from the STP (0C and 1 atmosphere pressure) figure quoted above.

i.e. speed of sound at temperature T is: vT = 331.45 + 0.59T [T is in degrees Celsius]

Sound is the term usually applied to the range of frequencies the human ear is capable of detecting (approximately50 hertz to 20 kilohertz). In a physical sense frequencies outside this range have properties in common with audiblesound waves, including the longitudinal wave nature. The term ultrasound is used for sounds with a frequencygreater than 20 kHz. Infrasound is a term sometimes applied to sounds with a frequency of less than 50 Hz.

Problem: Calculate the speed of sound at a temperature of 30 C. (Ans: 349 ms 1)

*id you !now+ E ephants communicate over distances of more than ,- !m using infrasound) E ephants put a ot of energy into these waves and they trave great distances because of this) The fre.uencies are too owfor humans to hear) Crocodi es a so communicate using infrasound)

InvestigationDetermine the speed of sound. (see Appendix 2 )



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The following is a diagram of an oscilloscope screen.

It comes from a website

http://plabpc.csustan.edu/general/GeneralPhysicsIIlabs/Oscilloscope/Oscilloscope.htm

The vertical scale is an arbitrary one. The horizontal scale can be used to calculate the period and hence thefrequency of the wave as demonstrated in the following example.

In this case, the period of the wave isapproximately 3.4 time (horizontal)divisions.

This corresponds to a period of 3.4 x 1 msi.e. 3.4 x 10 3 s.

The frequency is the reciprocal of this.

Therefore, f = 1/(3.4 x 10 3 s)= 294 Hz.

Investigation

Use a CRO and microphone to investigate

frequency (including the measurement of the unknown frequency of a sound)

amplitude

complex sound waves from different sources

Use a data logger with microphone input to investigateechoes

complex waves (for a Fourier analysis, use a Vernier LabPro with computer interface)

Ana ysis of SoundsThe following graphs show some sounds analysed using Audacity (Computer-based sound analysis similar toCRO). The sounds were produced using a flute, which is an instrument that produces a relatively pure tone.Audacity uses a microphone attached to the computer to covert the sound waves to electrical signals.

Sound waves are longitudinal waves.

A cathode ray oscilloscope with a microphone connected to its input can be used to show how pressure variations

associated with sound vary with time. Many computer-based software programs which perform the functions of aCRO one is called Audacity. e.g. of the record of a fairly pure sound produced from a flute, obtained usingAudacity.



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Sound 1

Sound 2

The horizontal scale (Time/Div setting) is the same for both sounds. The vertical scale (Voltage/Div) is also thesame. A microphone connected to the CRO converts sound energy to a voltage, which is detected by the CRO.

Sound 2 has a lower frequency and less energy than sound 1.



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3re.uency and 'itch of Sounds Definitions

Frequency the rate of repetition of a periodic disturbance measured in hertz (Hz) (vibrations, or cycles, persecond)

Pitch Pitch is the subjective property of a tone, related to the frequency of the sound. The ear/braininterprets frequency in a relative way. Nevertheless, a simple relationship is observed the higherthe frequency of a sound, the higher the pitch of the perceived sound. Very few people haveperfect pitch the ability to recognise the true frequency of a sound in isolation.

Amp itude and Vo ume Definitions

Amplitude The amplitude of a wave is the maximum displacement of an oscillating particle from itsequilibrium position. For sound waves the amplitude is usually much smaller (typically less than amillimetre) than the wavelength (which ranges from centimetres to metres for audiblefrequencies).

Volume or loudness is the subjective property of a sound relating to its perceived loudness. It isdependent on a number of factors, including

the energy of the sound the more energy a sound has at a given frequency, the louder it willbe to the listener. The energy of a sound wave increases as the amplitude increases.

the frequency of the sound the ears sensitivity to sound is not the same across allfrequencies. Sounds with frequencies to which the ear is not very sensitive may be perceivedas quite soft, even if the energy possessed by the sound waves is large. Outside of the range50 Hz to about 20 kHz, humans cannot hear sound waves, regardless of their intensity(energy content)

At any given frequency, the volume of a sound increases with the amplitude of the sound vibration. Differentpitches may be perceived to have different volumes despite their having the same amplitude. Conversely, soundswith a different frequency may have the same subjective loudness, even though the amplitudes are different.

The above graph shows that human ears are most sensitive to sounds with frequencies in the range 1000 Hz to2000 Hz. What do you think the reason for this is? What sounds have we evolved with to which sensitivity is asurvival advantage?

Sound Leve sSound levels are measured in decibels, which relates the intensity of the sound to a reference level (10 12 Wm 2). Thereference level corresponds to the human threshold of hearing at the frequency to which our ears are most sensitive.The scale is a logarithmic one but even without a mathematical interpretation, it is wise to be aware of soundlevels, particularly the potential of loud sounds to damage hearing permanently. Particular caution should beexercised in relation to sound exposure. Ear protection should always be worn when operating noisy machinery. Asa guideline, if a person has to shout at you to be heard over a sound, its time to protect your hearing. In-earheadphones are a particular risk, as the sound pressure level on the eardrum can easily exceed safe limits. Whenwearing these headphones, if you cant hear a normal conversation near you, the sound may be damaging yourhearing.



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Every 20 decibels represents a tenfold increase in the sound pressure level.

Threshold of hearing = 0 dB SPL

Threshold of feeling = 120 dB SPL

Threshold of pain = 130 dB SPL

5$ dB one 1uarter 5/ dB 6 one hal' dB 6 no change

/ dB 6 t-ice $ dB 'our times dB 6 ten times

7 dB 6 one hundred times / dB 6 one thousand times * dB 6 ten thousand times

Australia has workplace regulations regarding exposure to sound levels. These are for workers protection. Theyshould be adhered to at all times.



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/ef ection of Waves Definition : Reflection occurs when energy such as sound or light strikes a boundary and the direction of propagation of the wave changes as a result of the interaction with the boundary.

A boundary may be

A change from one medium to another e.g. air to water

A change in the density of a medium e.g. warm air (lower density) cold air (higher density) boundary

A change in the geometry of a medium in which a wave is travelling e.g. even though the end of an organ pipeis open to the air, some of the energy of a sound wave inside the pipe upon reaching this opening will bereflected back into the pipe. Without this, the pipe would not produce a sustained sound.

There are two laws of reflection applicable to a simple ray or one-dimensional wave

1. The angle of incidence equals the angle of reflection from a plane surface

2. The incident and reflected wave propagation directions are in the same plane as the normal to the reflectingsurface. This is illustrated in the following diagram.

EchoesAn echo occurs when a sound wave is reflected from a boundary. Echoes play an important role in acoustics inbuildings and in our perception of music. Some amplifiers create echoes artificially to re-create the ambience of aparticular acoustic setting, such as a stadium or concert hall.

Echoes are used by animals in the natural world to both navigate and to locate prey. Bats are probably the mostfamous of the species that use echolocation. Some bird species use echolocation, as do dolphins.

Humans use echoes in medical ultrasononography. Radar locates objects such asaircraft by detecting a reflected radio signal. Sonar, used by ships and submarinesrelies on the reflection of sound.

Question: A ship sends a sound wave pulse towards the ocean floor. The reflectedpulse is detected 2 seconds later. What is the distance from the ship the ocean floor?The speed of sound in water is 1496 ms 1.

Absorption and Transmission of SoundTwo important phenomena occur when a sound wave strikes an interface betweentwo different media, for example travelling from air to water. At the boundary ,some of the energy is reflected and some is absorbed, thus travelling into the secondmedium.

The amount of energy reflected and absorbed depends on the properties of eachmedium. As a general rule, the more different the two media, the greater will be the

amount of reflected energy. Part of the science of acoustics deals with such properties of materials and the effect onsound waves. The medical application of ultrasound relies on the reflection of ultrasound at the boundary betweendifferent types of tissue.

A knowledge of this area of science plays a key role in the design of many everyday things including concert halls,

buildings, offices, furnishings, flats and houses, cars, aircraft and factories.The principles are also important in medicine in the areas of ultrasound and other imaging technologies. In the caseof ultrasound, a special gel is applied to the patient to increase the proportion of ultrasound energy entering thebody.



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Ultrasound is emitted by some cameras to aid in their auto-focussing on a particular subject. Ultrasound is also usedby range finding sensors on electronic data logging equipment.



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&easuring the speed of sound with a data oggerThe following apparatus was used to measure the speed of sound using a data logger with a microphone sensor.

The microphone detects a sound produced at the microphone end of the tube and then the echo (reflection) isdetected when it reaches the microphone after travelling to the closed end and back again. Here is the recordproduced using a 2 m length of tube.

In this case, the reflection is detected at 0.0116 seconds after the first of the sound is produced. The velocity is thus

given by v = s/t = 4/0.0116 = 345 ms1

.In this result, it can be seen from the amplitude of the reflected wave that most of the energy striking the tubesclosed end was reflected.



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'rincip e of Superposition

Two waves passing through the same region of space at the same time produce aresultant wave, which, at every point in the region of interaction, has a resultant amplitude, which is the sum of the amplitudes of the interacting waves.

After passing through the region of interaction, the two waves continue as if they had not interacted.Demonstration : Create a pure, high pitched (about 10 kHz) tone in a room. Move slowly around the room. Whatdo you observe about the sound? The sound is reflected from the walls of the room. Some reflected waves canceleach other out, while others augment each other. The resulting loudness at any particular point is due to thesuperposition of the reflected waves.

Given two or more waves, their summation is performed as a simple algebraic addition of their amplitudes at anytime and position. Additional reference: See file Waves and FFT.pdf 3 for complete discussion.

The superposition of two waves is shown in the following diagrams. The two components are shown in the diagramon the left. The components and their sum is shown in the diagram on the right.

Components

-2.5000

-2.0000

-1.5000

-1.0000

-0.5000

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

0 5 10 15

Series1

Series2

Superposition

-3.0000

-2.0000

-1.0000

0.0000

1.0000

2.0000

3.0000

0 5 10 15

Series1

Series2

Series3

Investigation

Use the Excel spreadsheet Superposition to investigate the addition (superposition) of two waves andcompare this with the results combining two or more pure tones and using a data logger with microphone inputor CRO to examine the resultant waveforms

Use the Vernier LabPro data logger and the Logger Pro software to determine the frequencycomponents of a complex wave (produced by say a guitar cord or cord on a piano) and reconstruct the soundfrom the components.

Applications

A laser beam produces an intense light because all the components waves (a) have the same wavelength andamplitude and (b) are all in phase with each other. When all of these component waves superimpose, the resultantwave is very energetic. This type of superposition of effects is called constructive interference .

Ordinary light sources such as flames, fluorescent or incandescent lights produce waves having a range of wavelengths, and phase relationships. This results in many of the waves cancelling each other out an effect whichis referred to as destructive interference .

In some expensive motor vehicles, and some factories, loudspeakers are used to create sound waves that cancel outunwanted sounds by creating a wave out of phase with the unwanted sound.

Superposition is essential in analogue radio and television communication since the signal (voice, music or picture)is added to a carrier signal. The resulting amplitude of the wave is the sum of the components.

The reverse of the process of superposition of waves (synthesis) is called Fourier analysis. This is anextraordinarily useful process in the analysis of complex signals. Fourier analysis plays a key role incommunication technology, as well as being an essential tool in the analysis of all sorts of data, from signalsreceived from distant galaxies to the analysis of data used to produce medical images in processes such as magneticresonance imaging (MRI).

3 From web site http://www.cord.edu/dept/physics/p128/lecture99_34.html

http://www.cord.edu/dept/physics/p128/lecture99_34.htmlhttp://www.cord.edu/dept/physics/p128/lecture99_34.htmlhttp://www.cord.edu/dept/physics/p128/lecture99_34.htmlhttp://www.cord.edu/dept/physics/p128/lecture99_34.html


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'hysics Sy abus Section ()#)4 15pg2



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*etection and 6ses of E ectromagnetic WavesThe following table summarises some of the methods used to detect different forms of electromagnetic radiation,and some of the uses made of electromagnetic waves.

Type of Radiation Method of Detection Uses

gamma5rays

8amma ray camera (medical)

8amma ray telescope

8eiger counter

Sterilising medical e1uipment

Medical diagnoses

Astronomical investigations

9adiation monitoring

:5rays;5ray 'ilm< electronic sensors

Sterilising medical e1uipmentMedical diagnosis and imaging

ultraviolet .hotographic 'ilm< solid state sensors 0luorescent lights< ,lacklight signatures9 cameras using a CC? (ChargedCouple ?evice @ an electronic device used in videocameras)< >9 sensitive diode (electronicsemiconductor device 5 used in remote controlreceivers on video audio e1uipment

&emperature measurement

Surveillance

micro-aves Micro-ave antenna tuned receiver Cell phones< cooking< satellite communication

radio -aves Antenna tuned receiver Communication< astronomy

See separate document >ntroduction to EM Waves!docfor more useful information about uses and detection.

The wave e.uationAs with all types of waves, the relationship between the three quantities, velocity, frequency and wavelength, forelectromagnetic waves is given by

where

Question: Calculate

(a) the frequency of 3 cm microwaves. (assume their speed = 3 x 10 8) [Ans. 10 GHz)(b) the wavelength of 2MMM (frequency 104.9 MHz) [Ans. 2.86 m](c) the wavelength of microwaves (from an oven). f = 2450 MHz [Ans. 0.122 m](d) the wavelength of mobile telephone electromagnetic waves, f = 1800 MHz. [Ans. 0.1667 m]

6ses of e ectromagnetic wavesDiscussion points

the place of electromagnetic radiation in the natural world

the use of electromagnetic radiation by humans

Electromagnetic radiation plays a key role in human communication.

In the natural world, our eyes have evolved to be sensitive to light, the part of the spectrum the Sun produces at thegreatest intensity. Our sense of sight plays a very significant part in communication with other humans throughnon-verbal signals as well as written information.

The technological application of electromagnetic waves to communication now dominates the way wecommunicate globally. Radio and television waves are used in the forms of communication that bear their names.Microwaves are used for mobile telephone communications, as well as for the transmission of television signals torelay points from the main transmitter. Microwaves are also used for communication with satellites.

Light and infrared radiation are used to transmit digital information through optical fibres. Most of todays globalcommunication, in terms of the amount of data, takes place through optical fibres at some point in thecommunication process.



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Light7 the eye and evo utionAn interesting connection exists between the sensitivity of the eye to light from the Sun and the process of evolution. When the first rudimentary eyes began to evolve, they were not very efficient at gathering light (or anyother form of electromagnetic radiation). Measurements show that the Suns energy is emitted most strongly in thevisible light part of the spectrum. When animals began to evolve sensitivity to electromagnetic waves, those thatcould respond best to survived and proliferated. The animals whose eyes performed best were those with eyes tothe brightest part of the spectrum visible light.The image to the right is from a web site

[http://www.bc.cc.ca.us/programs/sea/astronomy/light/lighta.htm ]

Different colours are related to different frequencies of light. Manytextbooks state that colour is dependent on wavelength, which is partlycorrect since the two quantities are related. In any given medium, red lighthas the longest wavelength of the visible frequencies and violet has theshortest. However, red light passing from air to water slows down, and thewavelength becomes less, but neither the colour nor the frequency change.

'roperties of Light Light travels is straight lines through any homogeneous medium. A homogeneous medium is one in which

the properties of the medium are independent of the position in the medium. Examples of inhomogeneousmedia include window glass which produces distortions as we look through it, the way heat haze affectslight passing through it and the atmosphere which makes stars, particularly those near the horizon, twinkle.Because light travels in a straight line, objects appear to be in the position, the direction from which the lightenters the eye. This means that the reflection of an object in a mirror makes the object look as though it isbehind the mirror.

Light is a part of the electromagnetic spectrum. The wavelengths range from the shortest for violet light (4000angstroms) to the longest for red light (7000 angstroms). Red light has the longest wavelength. Light of different frequencies (and hence wavelengths) are seen as different colours - the familiar range of spectralcolours is shown in the diagram below.

The nature of e ectromagnetic wavesElectromagnetic radiation is the result of two transverse waves one electrical and the other magnetic in nature vibrating at right angles to each other and also at right angles to the direction of travel. The diagram below showsone way of representing light as such a pair of transverse waves.

Reference for diagram modified from:

http://www.md.huji.ac.il/spectroscopy/chem-ed/light/em-rad.htm

http://www.md.huji.ac.il/spectroscopy/chem-ed/light/em-rad.htmhttp://www.md.huji.ac.il/spectroscopy/chem-ed/light/em-rad.htm


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The following diagram shows another method of representing an electromagnetic wave. In this case, only theelectric field component of the wave is shown.

E ectromagnetic Waves and &atterElectromagnetic waves interact with matter by being reflected , absorbed or transmitted . As a general rule, theshorter the wavelength of the electromagnetic radiation, the greater capacity it has to penetrate materials, notablysolids and liquids. Hence X-rays and gamma rays can penetrate materials readily, making them useful for revealinginternal structures otherwise hidden inside materials. These internal structures can only be seen using these formsof radiation providing the material is non-homogeneous, as is the case when bones are imaged using X-rays.

Wilhem Roentgen invented the X-ray tube. He decided that it should not be patented, thus making this inventionfreely available for all to use. One of his earliest photographic plate from his experiments was a film of his wife,Bertha's hand with a ring, was produced on Friday, November 8, 1895.

A modern X-Ray imageGamma rays, which are more penetrating than x-rays, can be used to see inhomogeneities in solid materials such asaircraft bodies and engine parts.

The general rule stated above has many exceptions. Radio waves, the least energetic form of electromagneticradiation, nevertheless travel readily through bricks and concrete, materials which stop the more energetic lightrays. The Earths atmosphere absorbs the most energetic forms of electromagnetic radiation, UV, X-rays andgamma rays. The less energetic light and radio waves pass readily through the atmosphere. Glass is transparent tolight and infrared rays, but opaque to ultra violet rays.

Absorption of ight and other e ectromagnetic radiation by matterElectromagnetic radiation interacts with matter in a variety of ways.

Reflection . Reflection of light makes objects visible. Reflection may be either specular (as in the case of

mirrored surfaces) or diffuse (resulting in scattering of light, as happens with reflection from most objects,giving them a non-shiny appearance).

Transmission . Objects may transmit light of particular wavelengths without any interference. When thisoccurs we say that the material is transparent to that wavelength. e.g. the skin is opaque to visible light butmoderately transparent to infra-red light and very transparent to X-rays. Concrete is transparent to radio waves.

When light moves from one transparent medium to another its direction of travel may change a processcalled refraction . This change of direction is caused by a change in velocity.

Absorption . Many materials absorb the energy from light. Colours of objects are the result of differentproportions of the light being reflected at different wavelengths while other wavelengths are absorbed. e.g. redobjects appear red because they reflect the wavelength of light that the eye perceives as red and absorb otherfrequencies.

The absorption of light and other forms of electromagnetic rays by a medium is dependent on

The frequency (and hence wavelength) of the radiationThe nature of the medium in which the wave is travellingThe distance the light travels through the medium



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Electromagnetic radiation may become polarised when it interacts with matter, or the plane of polarisation may bechanged (polarisation is not dealt with in this course it is an interesting phenomenon with applications as diverseas the vision of bees and the efficient transmission and reception of television signals)

The absorption and reflection of light from a material gives rise to colour. A material that absorbs all wavelengthsof light except red, which is reflected appears red when we look at it since we see the object as a result of the redlight it reflects. A surface that reflects both red and blue appears to be magenta in colour. The interaction of lightand the eye is very important since it gives us our sense of colour vision. Not all animals have the ability to seecolour.

It is interesting to note that there are two quite different ways in which we combine colours to form other ones. Thetwo processes are called additive and subtractive . On a television screen, video display, or in theatrical stagelighting different colours are produced by combining components in an additive process. A colour display screenreproduces the full range of spectral colours using just three primary colours (red, blue and green). The effect of adding these in different combinations is shown below (left) (view the pixels on your computer screen with amagnifying glass to see the individual pixels and component colours). In this process, the screen itself is the sourceof light.

Printing with inks, or painting however is a subtractive process. This is represented by the diagram above (right).The simplest colour printer uses three colour inks, magenta, cyan and yellow. Since magenta reflects blue and red(absorbing green), and cyan reflects blue and green (absorbing red), the only colour that is reflected in commonfrom a combination of these is blue the resulting colour when they are mixed. In practice, since pure pigmentcolours are difficult to obtain, the process is much more complex. The addition of cyan, yellow and magenta shouldproduce black, but it does not do so in practice and so even the cheapest colour printers now are four colour which means they use black pigment to ensure better reproduction of tones, especially black. In this process, thesource of light is not the pigment it is merely reflecting the light from another source. The colour of pigments canchange dramatically when viewed under different lighting conditions.

There are no simple rules to predict the reflection and absorption of other components of the electromagneticspectrum by different materials. A brick wall that is totally opaque to visible light, is almost perfectly transparent toradio waves. Metals, which totally absorb radio waves, allow gamma rays to pass through them quite readily.

Glass with a high lead content allows radiographers to see their patient, because light is transmitted through theglass, but it blocks x-rays, which are absorbed by the lead, protecting the radiographer.

The development of high purity glass, which hardly absorbs any electromagnetic radiation in the infrared region of the spectrum has given rise to optical fibre based communication s technology.

When a material absorbs electromagnetic radiation, the energy produces changes in the material as the absorbedenergy is converted to other forms of energy. Light energy is absorbed by chlorophyll in plants, and the lightenergy causes chemical changes, which result in the formation of glucose (a high-energy compound). In filmphotography, light produces chemical changes in the chemicals on the film, which eventually allows an image to beproduced. Infrared radiation causes molecules to vibrate when it is absorbed, thus heating materials up. A TVantenna absorbs the energy of the TV signal, converting it to an electric current in the antenna.

The absorption of different forms of electromagnetic radiation by the atmosphere plays a vital role in protecting lifeon Earth from gamma rays, x-rays and ultraviolet rays, all of which can destroy life. The opaqueness of theatmosphere to these wavelengths has until recently frustrated astronomers in their investigation of the Universe.Satellites above the atmosphere now allow us to see the Universe using these wavelengths.



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The vertical bars extending downward from the top of the chart show the height at which about half of the radiationentering the atmosphere has been absorbed as it travels through the air.



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Absorption of E ectromagnetic /adiation by Earth8s AtmosphereAll forms of electromagnetic radiation reach the Earths atmosphere from stars, nebulae, matter surrounding black holes, stellar and galactic collisions and other unknown sources. The Earths atmosphere plays a vital role infiltering the electromagnetic radiation from space. Consider the following graphic [Source:http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html ] which shows the relative absorption of

different parts of the electromagnetic spectrum as the waves interact with the atmosphere at various altitudes.

Alternative Reference: spaceflt.pdf

The filtering out of UV, X-rays and gamma rays is essential for the continuance of life on Earth since these formsof radiation are all harmful to living organisms. In recent years, concern has developed over the apparentdestruction of the ozone layer by CFCs (organic chemicals, commonly used in air conditioners, fridges, medicalfields and the electronics industry). Ozone strongly filters UV radiation from the Suns rays. Without the ozone, life

as we know it on Earth would not be possible.Astronomers have long made use of light and radio waves reaching Earth from space to investigate the Universe.Since the atmosphere filters out UV, X-rays and gamma rays, it was not useful to try and build Earth baseddetectors of these forms of radiation. In recent years telescopes designed to see these high energy waves havebeen put into orbit around the Earth. These are currently yielding important new information about our Universe.Check out [ http://chandra.harvard.edu/ ]

These images of the astronomical object PKS 0637-752 are shown to the same scale, viewe b! "-ra! #left$ an light #right$ telesco%es& PKS 0637-72 is so istant that we see it as it was 6 billion !ears ago& 't is a l(mino(s )(asar that ra iates with the %ower of *0 trillion s(ns from a region smaller than o(r solar s!stem& The so(rce of this %ro igio(senerg! is believe to be a s(%ermassive blac+ hole& a io telesco%e observations of PKS 0637-752 show that it hasan e"ten e ra io jet that stretches across several h(n re tho(san light !ears& han ra.s "-ra! image ma e withthe / vance 'maging S%ectrometer #/ 'S$ reveals a %owerf(l "-ra! jet e"ten ing more than 200 000 light !ears into intergalactic s%ace that is %robabl! (e to a beam of e"tremel! high-energ! %articles&



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The Inverse S.uare Law for Light

FluxEnergy is measured in units called joules (J). The total flow of energy through a surface (imaginary or otherwise)can thus be measured in joules. This is called the flux of the radiation. Flux density is the energy flow through agiven area (square metre in the metric system) every second has units therefore of joules per square metre. The rateat which energy is used/transferred (measured in joules per second) is called power and the unit for power (Js 1) isgiven the special name a watt (W). Hence, the energy flowing through a surface, or falling on that surface, can bemeasured in watts/metre 2 (Wm -2).The energy from the Sun, reaching the Earths upper atmosphere is approximately 1400 Wm 2. At the surface of theEarth, this reduces to about 1000 Wm 2. The difference is due to reflection of energy back into space, andabsorption by the atmosphere with subsequent re-emission into space.The difference in the heating effect of theSuns rays on the Earth at different latitudesand in different seasons is due to variations inirradiance caused by the angle of the Earthssurface to the incoming rays. This isillustrated in the adjacent diagram.It can be seen that the same flux, in this caserepresented by five rays, at the equator fallson a smaller area at the equator than thatamount of flux at a higher latitude. Thuswhen the Sun is directly over the equator (theequinox), the flux density becomes less, thefurther it is measured from the equator.In this diagram, the Suns rays are drawn asbeing parallel to each other. This is a closeapproximation to the truth. The rays from theSun diverge of course, but because the Earthis relatively small compared with the distance from the Sun, the rays reaching the Earths surface are effectively

parallel. That is why objects cast fairly sharply defined shadows. It should be noted that if the object casting theshadow is very small compared with its distance from the surface, it casts a shadow with indistinct edges. This canbe observed if one looks at the shadow of a plane flying overhead, a bird or the shadows of overhead electricalwires.Light from a source spreads out as it propagates through the surrounding space. Light energy leaving a pointsource spreads out in a spherical pattern.

All of the energy passing through an imaginarysphere 1 m from the source will subsequently passthrough a sphere of radius 2 m, then 3, 4 m and soon. The surface area of a sphere is proportional to thesquare of the radius, since

SA = 4 R2

Hence, the energy passing through a sphere of radiusone metre is spread over four times the area when itpasses through a sphere 2 m away, and over ninetimes the area as it passes through the 3 m radiussphere. The flux density, is thus a quarter of itsvalue as it passes through a surface twice thedistance away, and just one ninth as it passes througha surface at three times the distance.This is illustrated in the following graphic.

The relationship between luminous flux density anddistance from the source is shown graphically in the following graph. Qualitatively, this relationship can bedescribed as As the distance from the source increases, the flux density decreases at a decreasing rate.Mathematically, the relationship between these variables is called an inverse square relationship.



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The inverse square law for light can thus be stated as

The luminous flux density from a source decreases with thesquare of the distance from the source.

The inverse square law has important implications in practical situations. Lighting design in buildings must takethis effect into account. Radiologists maximise their distance from x-ray machines when they are working, sincethis will minimise their exposure. Distance between people and sources of radiation in the event of a radiationrelated accident is of key importance in reducing exposure.

The inverse square law is a mathematical relationship occurring in many different contexts in physics, including therelationship between force of gravity between masses and distance, and forces between charged objects (due totheir charge) and distance.



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!ommunicating with %lectromagnetic Waves

http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html

&roadcast 'ignals

Information is broadcast using radio signals by producing an electromagnetic wa e that is the result of thesuperposition of a signal deri ed from a sound source! such as a microphone! onto a much higher fre"uency component called a carrier .

#adio communication is typically in the form of A$ radio or %$ #adio transmissions. &he broadcast of asingle signal! such as a monophonic audio signal! can be done by straightforward amplitude modulation or fre"uency modulation. $ore comple' transmissions use what are (nown as sidebands produced from thesum and difference fre"uencies resulting from the superposition of some signal upon the carrier wa e. %or e'ample! in %$ stereo transmission! the sum of left and right channels )*+#, is used to fre"uencymodulate the carrier and a separate subcarrier at (H is also superimposed on the carrier. &hat sub-carrier is then modulated )superposition, with a )*-#, or difference signal so that the transmitted signal can

be separated into left and right channels for stereo playbac(. In tele ision transmission! three signals must be sent on the carrier: the audio! picture intensity! and picture chrominance. &his process ma(es use of twosub-carriers. 0ther transmissions such as satellite &1 and long distance telephone transmission ma(e use of multiple sub-carriers for the broadcast of multiple signals simultaneously.

A( Radio

2hen information is broadcast from an A$ radio station! the electrical image of the sound )ta(en from amicrophone or other program source, is used to modulate the amplitude of the carrier wa e transmittedfrom the broadcast antenna of the radio station. &his is in contrast to %$ radio where the signal is used tomodulate the fre"uency of the carrier.

&he A$ band of the Electromagnetic spectrum is between 3 3 4H and 5673 (H and the carrier wa esare separated by 8 (H in Australia and 57 (H in many other countries! including the 9SA.

A radio recei er can be tuned to recei e any one of a number of radio carrier fre"uencies in the area of therecei er. &his is made practical by transferring the signal from the carrier onto an intermediate fre"uency inthe radio by a process called heterodyning. In a heterodyne recei er! most of the electronics is (ept tuned tothe intermediate fre"uency so that only a small portion of the recei er circuit must be retuned whenchanging stations.

A$ radio uses the electrical image of a sound source to modulate the amplitude of a carrier wa e. At therecei er end in the detection process ! that image is stripped bac( off the carrier and turned bac( into sound

by a loudspea(er .

http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.htmlhttp://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/mic.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/ems1.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/sumdif.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/amfmdet.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/spk.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.htmlhttp://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/mic.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/ems1.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/sumdif.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/amfmdet.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/spk.html#c1


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F( Radio

2hen information is broadcast from an %$ radio station! the electrical image of the sound )ta(en from amicrophone or other program source, is used to modulate the fre"uency of the carrier wa e transmittedfrom the broadcast antenna of the radio station. &his is in contrast to A$ radio where the signal is used to

modulate the amplitude of the carrier.%$ radio has a greater bandwidth than A$ in the commercial fre"uency ranges used for each and this ledto its early use for broadcasting stereo radio. %$ tends to be less affected by electrical interference such asstorms. A$ stereo is now a ailable! howe er it is not widely used in Australia and will probably besuperseded by digital stereo radio.

&he %$ band of the electromagnetic spectrum is between $H and 57 $H and the carrier wa es forindi idual stations are separated by 77 (H for a ma'imum of 577 stations. &hese %$ stations ha e a ;3(H ma'imum de iation from the centre fre"uency! which lea es 3 (H upper and lower acent fre"uency band. &his separation of the stations is much wider thanthat for A$ stations! allowing the broadcast of a wider fre"uency band for higher fidelity music broadcast.It also permits the use of sub-carriers which ma(e possible the broadcast of %$ Stereo signals.

%$ radio uses the electrical image of a sound source to modulate the fre"uency of a carrier wa e. At therecei er end in the detection process! that image is stripped bac( off the carrier and turned bac( into sound

by a loudspea(er .

!ommunication &ands

&he radio spectrum is di ided into fre"uency bands! the use of which is go erned by internationalagreements. *icences are re"uired to transmit signals in most of the bands.

Low Frequency (LF) - 7 4H to 77 4H although there are signals transmitted well below this region

principally the 0$E?A na igation networ(.Medium Frequency (MF) - 77 4H to $H which mainly includes the A$ radio band of about 3 74H to 5637 4H ) aries between countries,.

High Frequency (HF) - $H to 7 $H and comprises amateur radio! short wa e broadcasters among ahost of others. *argely becoming superseded by satellite transmissions.

Very High Frequency (VHF) - 7 $H to 77 $H occupied by traditional &1 stations! some amateur bands! commercial two way radio! maritime and aircraft bands as well as the %$ radio band of - 57$H .

Ultra-high Frequency (UHF) - 77 $H to ?H this band is occupied by 9H% &1! some radar

installations! mobile phones! two-way radios.Beyond 3 H! is mainly used for satellite transmissions.

http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/mic.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/ems1.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/ems1.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/sumdif.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/amfmdet.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/spk.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/mic.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/bcast.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/ems1.html#c1http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/radio.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/sumdif.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/amfmdet.html#c2http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/spk.html#c1


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It is interesting to note by way of numerical comparison that firstly! each band is 57 times the pre ious band. Secondly the *% band spanning 7 to 77 4h could be duplicated 57!777 times o er in the spaceoccupied by the 9H% band.

Also at the bottom end of 7 4h the signal cycle repeats 7 777 times a second. At the top of the 9H% band the signal cycle repeats 777 777 777 times a second )mind boggling eh@,.

(obile Phone Frequencies)'( ?lobal System for $obile ommunication. A communication standard in three fre"uency bands! 877 $H !5 77 $H and 5877 $H . &he term ?S$ is often used in A9SA*IA to refer to the 877 $H bandwhile the 5 77 $H band is referred to as C. &he ?S$ Association is responsible for the de elopment!deployment and e olution of the ?S$ standard.

)'(*++ A networ( which operates in the 877$H ?S$ band.

)'(,-++

A networ( which operates in the 5 77$H ?S$ band.)'(,*++ A networ( that operates in the 5877$H ?S$ band. Some networ(s in the 9SA! South America! Asia andAfrica use this band.

What is .&andwidth/0

(efinitions of Bandwidth on the Web)

A measure of the capacity of a communications channel. &he higher a channelDs bandwidth! themore information it can carry.www.tamu.edu/ode/glossary.html

&he amount of information or data that can be sent o er a networ( connection in a gi en period oftime. Bandwidth is usually stated in bits per second )bps,! (ilobits per second )(bps,! or megabits

per second )mps,.www.tecrime.com/7gloss.htm

A relati e range of fre"uencies that can carry a signal on a transmission medium.www.adapti edigital.com/ser ices/ser definitions.htm

Bandwidth is the amount of data that can be transferred o er the networ( in a fi'ed amount oftime. 0n the Cet! it is usually e'pressed in bits per second )bps, or in higher units li(e $bps)millions of bits per second,. . modem can deli er ! 77 bps! a &5 line is about 5.3 $bps.www.hosttrail.com/glossary/b/

A measure of spectrum )fre"uency, use or capacity. %or instance! a oice transmission by telephonere"uires a bandwidth of about 777 cycles per second ) 4H ,. A &1 channel occupies a

bandwidth of 6 million cycles per second )6 $H , in terrestrial Systems. In satellite based systemsa larger bandwidth of 5;.3 to ; $H is used to spread or


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unreadable.www.iec-usa.com/Browse7 /?*SB.html

http://www.google.com.au/url?sa=X&start=6&oi=define&q=http://www.iec-usa.com/Browse02/GLSB.htmlhttp://www.google.com.au/url?sa=X&start=6&oi=define&q=http://www.iec-usa.com/Browse02/GLSB.html


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Communicating with E ectromagnetic WavesElectromagnetic waves are used to transmit information. This information may be from sound, including voices andmusic, or it may be from images or digital data from a computer.

The principle is the same for all of these. A particular frequency of electromagnetic wave has one or more of itsproperties altered slightly by combining a pure carrier signal with the information being sent. This may be asimple case of the superposition of the information onto the carrier, producing a resultant waveform. This is anapplication of the principle of superposition. Digital communications technology involves the switching on and off of a wave, representing zeros and ones, which can then be used to encode information.

Amplitude a nd Frequency Modulation

The important thing in any communications system is to be able to send information from one place to another.This means we have to find a way to impress that information on the radio wave in such a way that it can berecovered at the other end. This process is known as modulation. In order to modulate a radio wave, we have tochange either or both of the two basic characteristics of the wave: the amplitude or the frequency.

AM : Amplitude Modulation

If we change the amplitude, or strength, of the signal in a way correspondingto the information we are trying to send, we are using amplitude modulation,or AM. The earliest means of radio communications was by Morse code, andthe code key would turn the transmitter on and off. The amplitude went fromnothing to full power whenever the key was pressed, a basic form of AM.

Modern AM transmitters vary the signal level smoothly in direct proportion tothe sound they are transmitting. Positive peaks of the sound producemaximum radio energy, and negative peaks of the sound produce minimumenergy.

The main disadvantage of AM is that most natural and man made radio noiseis AM in nature, and AM receivers have no means of rejecting that noise.Also, weak signals are (because of their lower amplitude) quieter than strongones, which requires the receiver to have circuits to compensate for the signallevel differences.

FM" FrequencyModulationIn an attempt to overcome these problems, a man named Edwin H.Armstrong invented a system that would overcome the difficulties ofamplitude noise. Instead of modulating the strength (or amplitude) ofthe transmitted signal, or carrier, he modulated the frequency.Though many engineers at that time said that FM was not practical,

Armstrong proved them all wrong, and FM today is the mainstay ofthe broadcast radio services.

When information is encoded using frequency modulated, thefrequency of the carrier wave is varied according to the modulatingsignal. For example, positive peaks would produce a higher

frequency, while negative peaks would produce a lower frequency. At the receiving end, a limiting circuit removesall amplitude variations from the signal, and a discriminator circuit converts the frequency variations back to theoriginal signal.

In this way, the effects of amplitude noise are minimized. Since the recovered audio is dependent only on thefrequency, and not the strength, no compensation for different signal levels is required, as is the case with AMreceivers.



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Digital EncodingIn this process the information is converted to a string of binary numbers, zeroes and ones, which are transmittedusing light (optical fibre telephone transmission of voice and internet), microwave (cellular phone) or television(soon to be introduced in Australia) electromagnetic waves. When information is digitally encoded, the signal has

just two states, usually called on and off. Once the information has been encoded, a digital stream of data looksmuch the same regardless of whether it is computer data, sound information or pictures.

Digital communication is now a key form of communication, affecting personal communication, TV as well as allcomputer data on the Internet and all the essential data associated with global economic transactions.

Limitations of Communication 6sing E ectromagnetic WavesThe following table summarises the application of various forms of electromagnetic radiation to communicationstechnology and includes some of the limiting characteristics of each.

Type of /adiation Communication 6se Limitations of this Techno ogy

gamma5rays +ot used &oo high a 're1uency 'or electronic detection andprocessing! &oo penetrating! ?angerous!

:5rays +ot used &oo high a 're1uency 'or electronic detection andprocessing! &oo penetrating! ?angerous!

ultraviolet Some 're1uencies used in optical 'i,re communicationstechnology

>t is di''icult to generate ultraviolet light using solid stateelectronics

visi,le sed in optical 'i,re communication technology @ voiceustments needed for accurate positioning. &herecei er uses the time difference between thetime of signal reception and the broadcast timeto compute the distance! or range! from therecei er to the satellite.The receiver must account for propagationdelays, or decreases in the signal's speedcaused by the ionosphere and the troposphere .With information about the ranges to threesatellites and the location of the satellite whenthe signal was sent, the receiver can computeits own three-dimensional position.An atomic clock synchronized to GPS is required in order to compute ranges fromthese three signals. However, by taking a measurement from a fourth satellite, thereceiver avoids the need for an atomic clock. Thus, the receiver uses four satellites tocompute latitude, longitude, altitude, and time.

*eve opment

GPS is available in two basic forms: the standard positioning service (SPS) and theprecise positioning service (PPS). SPS provides a horizontal position that is accurateto about 100 m. PPS is accurate to about 20 m. For authorised usersnormally theUnited States military and its alliesPPS also provides greater resistance to

jamming and immunity to deceptive signals.Enhanced techniques such as differential GPS (DGPS) and the use of a carrierfrequency processing have been developed for GPS. DGPS employs fixed stations onthe earth as well as satellites and provides a horizontal position accurate to about 3m. Surveyors pioneered the use of a carrier frequency processing to computepositions to within about 1 cm. SPS, DGPS, and carrier techniques are accessible toall users.

The availability of GPS is currently limited by the number and integrity of thesatellites in orbit. Outages due to failed satellites still occur and affect many userssimultaneously. Failures can be detected immediately and users can be notifiedwithin seconds or minutes depending on the users specific situation. Most repairs

http://find/Concise.asp%3Fz=1&pg=2&ti=761562277http://find/Concise.asp%3Fz=1&pg=2&ti=761556299http://find/Concise.asp%3Fz=1&pg=2&ti=761562277http://find/Concise.asp%3Fz=1&pg=2&ti=761556299


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are accomplished within one hour. As GPS becomes integrated into criticaloperations such as traffic control in the national airspace system, techniques formonitoring the integrity of GPS on-board and for rapid notification of failures arebeing developed and implemented.As of March 1994, 24 GPS satellites were in operation. Replenishment satellites areready for launch, and contracts have been awarded to provide satellites into the 21stcentury. GPS applications continue to grow in land, sea, air, and space navigation.The ability to enhance safety and to decrease fuel consumption will make GPS animportant component of travel in the international airspace system. Airplanes willuse GPS for landing at fogbound airports. Automobiles will use GPS as part of intelligent transportation systems . Emerging technologies will enable GPS todetermine not only the position of a vehicle but also its altitude.

Sate ites

GPS satellites fly in circular orbits at an altitude of 20,100 km and with a period of 12 hours. The orbits are tilted to the earth's equator by 55 degrees to ensure coverage

of polar regions. Powered by solar cells, the satellites continuously orient themselvesto point their solar panels toward the sun and their antennae toward the earth. Eachsatellite contains four atomic clocks.The control segment monitor the GPS satellites and uses measurements collected bythe monitor stations to predict the behavior of each satellite's orbit and clock. Theprediction data is uplinked, or transmitted, to the satellites for transmission to theusers. The control segment also ensures that the GPS satellite orbits and clocksremain within acceptable limits.The user segment includes the equipment of the military personnel and civilians whoreceive GPS signals. Military GPS user equipment has been integrated into fighters,bombers, tankers, helicopters , ships, submarines , tanks , jeeps, and soldiersequipment. In addition to basic navigation activities, military applications of GPSinclude target designation, close air support, smart weapons, and rendezvous.With more than 500 000 GPS receivers, the civilian community has its own large anddiverse user segment. Surveyors use GPS to save time over standard survey methods.GPS is used by aircraft and ships for en route navigation and for airport or harborapproaches. GPS tracking systems are used to route and monitor delivery vans andemergency vehicles. In a method called precision farming, GPS is used to monitorand control the application of agricultural fertilizer and pesticides. GPS is availableas an in-car navigation aid and is used by hikers and hunters. GPS is also used on theSpace Shuttle. Because the GPS user does not need to communicate with the

satellite, GPS can serve an unlimited number of users. [source: Research assignment by Alan Lam]

Some useful GPS linkshttp://www.howstuffworks.com/gps.htm

The above site is a very good outline of GPS operation with links, including

http://www.trimble.com/gps/howworks/aa_hw1.htm

The above site has an excellent detailed explana

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