8 Communication applications of EM waves · 8 Communication applications of EM waves 136 Wave me...

8 Communication applications of EM waves

136

Wave me helloWant to listen to a live concert broadcast from London? How are the stock prices on Wall Street? Who is winning the FIFA World Cup qualifier game in Rio de Janeiro? What is the weather like in Bangkok? For most Australians with a computer and Internet access, all of this is possible in minutes from the comfort of home and it is hard to imagine an Australia that was once isolated from information.

This isolation was ended by a communications technology revolution that used electromagnetic (EM) waves. This chapter describes the interesting properties of EM waves and how they have been used as the basis of modern communication technologies.

8.1 Properties of EM waves Electric and magnetic forces are said to act at a distance because charged

and magnetised particles produce regions of influence (or fields) in the space surrounding them. For example, iron filings placed near, but not touching, a permanent bar magnet will experience an attractive force (Figure 8.1.1). Similarly, charged Perspex and ebonite rods will repel and attract other charged objects without touching them.

A stationary charged particle will produce a three-dimensional stationary electric field in the space surrounding it, the field strength decreasing with distance from the charged particle. If the charged particle vibrates or oscillates, the corresponding electric field will also oscillate (Figure 8.1.2).

When a charged particle moves in space, it also exerts magnetic forces and possesses a magnetic field. Therefore, an oscillating charged particle produces an oscillating electric field and an oscillating magnetic field. These oscillating fields propagate together through the space around the charged particle at the speed of light. These two oscillating fields together are called electromagnetic (EM) waves (Figure 8.1.3).

Describe EM waves in terms of their speed in space and their lack of requirement of a medium for propagation.

field, refractive index, electromagnetic spectrum,

atmosphere, focus, focal point, focal length, diverge, image, critical angle, total internal reflection, optical fibre,

analogue, audio wave, amplify, digital, carrier wave, modulation, amplitude

modulation, frequency modulation, bandwidth, video wave, ionosphere,

geosynchronous satellite, digital technology, binary code

Figure 8.1.1 An invisible magnetic field surrounds the bar magnet and the iron fillings experience an attractive force.

THE WORLDCOMMUNICATES

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+ +

Electric field surrounding astationary positive charge

Electric field produced by a positivecharge oscillating up and down

a b

Figure 8.1.2 Electric field lines produced by (a) a stationary positive charge and (b) an oscillating positive charge.

The oscillations of the electric and magnetic fields are perpendicular to each other and both are at right angles to the direction of wave propagation. Hence, EM waves are transverse waves.

Refractive indexElectric and magnetic fields can be established in most media and in a vacuum; once the fields are established the EM wave will propagate. EM wave speed is greatest in a vacuum as there is no matter to absorb energy or distort the field lines. The speed of EM waves in a vacuum (c) is 300 000 km s–1 (3 × 108 m s–1). The wave speed changes when it travels through different types of matter as the electric and magnetic behaviour varies according to the physical properties of the matter (see Table 8.1.1). The ratio of the speed of an EM wave in a vacuum (c) to that in matter (v) is known as the refractive index (n):

n = cv

Table 8.1.1 Speed of light in some gases, liquids and solids

SUBSTANCE REFRACTIVE INDEX (n) EM WAVE SPEED (m s–1)Vacuum 1 2.9979 × 108

Gases at 0°C and 1 atm

Air 1.000293 2.9970 × 108

Carbon dioxide 1.000450 2.9965 × 108

Liquids at 20°C

Water 1.333 2.2490 × 108

Benzene 1.501 1.9970 × 108

Solids at room temperature

Diamond 2.419 1.2390 × 108

Crown glass 1.52 1.9700 × 108

Explain that refraction is related to the velocities of a wave in different media.

Figure 8.1.3 Perpendicular oscillating electric and magnetic fields produce electromagnetic (EM) waves.

electric field

magnetic field

direction ofmotion

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Electromagnetic spectrumAll EM waves are essentially the same in structure: they propagate according to physical laws that underpin electric and magnetic fields in a vacuum; they all travel at the same speed (c = 3 × 108 m s–1); and they are transverse waves. However, an oscillating charge that produces an EM wave can oscillate at different frequencies. This means EM waves come in a broad range of frequencies and we identify these different frequency waves by names like radio waves, microwaves, infra-red (IR), visible light, ultraviolet (UV), X-rays and gamma rays. The many possible variations in frequency produce a spectrum of EM waves called the electromagnetic spectrum.

X-rays

Aircraft andshippingbands

AM radio

Short-waveradio

TV andFM radio

MicrowavesRadar

Infra-redlight

Visible

Ultravioletlight

Gamma rays

Wavelength (m)

1 × 10–13 1 × 1022

1 × 10–10 1 × 1018

1 × 10–9 1 × 1017

1 × 10–6 1 × 1014

1 × 10–4 1 × 1012

1 × 10–2 1 × 10101 × 100 1 × 108

1 × 101 1 × 107

1 × 102 1 × 106

1 × 103 1 × 105

1 × 105 1 × 103

1 × 10–12

1 × 10–16

1 × 10–17

1 × 10–20

1 × 10–22

1 × 10–241 × 10–26

1 × 10–27

1 × 10–28

1 × 10–29

1 × 10–31

Frequency (Hz) Energy (J)

Figure 8.1.4 The spectrum of electromagnetic waves

From Figure 8.1.4, you will notice that high-frequency waves like X-rays have very short wavelengths and that low-frequency waves like radio waves have very long wavelengths. As previously described in Section 5.6, wave speed is dependent on wavelength and frequency:

v = f λ

and all EM waves travel at the same speed. This means that wavelength and frequency are inversely proportional for EM waves: as frequency increases, wavelength decreases and vice versa. The energy of the EM waves increases with frequency.


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Worked exampleQUESTIONCalculate the wavelength of the Triple J FM radio wave (in Sydney) with frequency 105.7 MHz.

SOLUTIONFrequency and wavelength are related according to the equation v = f λ.

v = c = 3 × 108 m s–1, f = 105.7 × 106 Hz

λ =

= ×

=

vf

m sHz

m

3 10105 7 102 8

8 1

6

−

×..

The wavelength of the Triple J radio wave is about 3 m (to 1 significant figure).

EM waves occur naturally and permeate the space around us. Stars, huge spheres of hot ionised gas, produce large amounts of EM waves that travel through the vacuum of space. It takes approximately 8 minutes for the EM waves produced by the Sun to reach the Earth’s surface; we are continually bombarded by EM waves of an extremely wide range of frequencies from the Sun. Other natural EM wave sources include radioactive atoms (such as uranium and caesium) in the Earth’s crust producing gamma rays, charged particles (like electrons) generating lower frequency EM waves, and hot bodies producing visible and IR waves.

In large amounts, the high-frequency high-energy EM waves from the Sun can damage the genetic material of living things. Luckily for us, the Earth is surrounded by a roughly 100 km thick layer of gas molecules and ions called an atmosphere. Radio waves and light easily penetrate the atmosphere and make it to the surface; however, some IR and nearly all of the high-frequency EM waves (UV, X-rays and gamma rays) are either absorbed or reflected by the atmosphere and never reach the surface.

Information relating to the applications and detection methods of different types of EM waves is provided in Table 8.1.2 on page 140.

Solve problems and analyse information by applying the mathematical model of v = f λ to a range of situations.

Identify EM wavebands filtered out by the atmosphere, especially UV, X-rays and gamma rays.

HUBBLE SPACE TELESCOPE

Astronomers refer to the Earth’s atmosphere as a ‘ceiling’ with an ‘optical window’ and ‘radio

window’. This means that light and radio waves from space can be easily observed from the Earth’s surface using telescopes but that UV, X-ray and gamma ray sources are undetectable. Telescopes mounted on satellites above the Earth’s atmosphere can collect EM waves of both high and low frequency. The Hubble Space Telescope is fitted with cameras sensitive to IR, visible and UV radiation, and has produced images and information about the universe that would have been impossible to collect from Earth.

Figure 8.1.5 Image of the Cone Nebula taken using a camera on the

Hubble Space Telescope, which is sensitive to IR, visible

and UV radiation.


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Table 8.1.2 Applications and detection methods of different types of EM waves

EM WAVE APPLICATIONS DETECTION METHODSRadio waves Television•

FM Radio•

AM Radio•

Antennas/aerials with radio • frequency detection circuit

Microwaves Mobile phones•

Wireless data transfer•

Microwave ovens•

Garage door openers•

Aviation guidance systems•

Speed-checking radar•

Antennas/aerials with radio • frequency detection circuit

Materials that fluoresce • (glow) when exposed to microwaves

IR Space heater•

Ovens•

TV remote control•

Night vision goggles•

Thermoreceptor cells in • animal skin

Thermocouples•

Electronic photo-detectors•

Light Vision•

Photography•

Plant photosynthesis•

Lasers•

Lighting•

Photoreceptor cells in eyes•


Light meters•

Photographic film•

Photomultipliers•

UV Security scanning of documents•

Solariums•

UV curing of polymers•

Sterilisation•

Materials that fluoresce • (glow) when exposed to UV


Photomultipliers•

X-rays Medical diagnosis and treatment•

Security screening•

Screening for metal corrosion and • structural weakness

X-ray film•

Electronic detectors and • counters

Geiger counters•

Gamma rays Medical diagnosis and treatment•

Security screening•

Sterilisation•

Screening for metal corrosion and • structural weakness

Geiger counters•

Thermoluminescent • detectors

X-ray film•

CHECKPOINT 8.11 Outline how EM waves propagate without a medium.2 Describe the relationship between EM wave speed and the refractive index.3 List four types of EM waves that reach the Earth’s surface in reduced amounts due to filtering by the atmosphere.


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8.2 EM wave reflectionThe law of reflection (described in Section 6.6) applies equally to all EM waves. The angle of an incidence ray equals the angle of a reflected ray:

θi = θr

We have also described how waves reflect from flat (or plane) surfaces; however, many applications utilising EM waves have curved surfaces, such as satellite dishes or the concave and convex mirror surfaces. Waves still obey the law of reflection on encountering curved surfaces: each ray from the source strikes a point on the curved surface and is reflected, such that θi = θr. The normal line from which the angles θi and θr are measured is perpendicular to a tangent at this point (Figure 8.2.1).

incident ray

normal

reflected ray

incident ray

normal

reflected ray

i r

Concave mirrora

tangent

Convex mirrorb

tangent

i

r

Figure 8.2.1 Reflection from curved surfaces: (a) concave mirror; (b) convex mirror

A reflective concave surface will tend to concentrate (or focus) the rays. An ideal concave mirror has a parabolic surface because parabolas focus parallel incident rays to a single point called the focal point. However, a spherical mirror with shallow curvature is a good approximation to a parabola. The distance between the focal point and the reflective surface is called the focal length. A reflective convex surface will tend to spread (or diverge) the rays (Figure 8.2.2).

parallel incident rays parallel incident rays

focal length focal length

b Concave mirror

focalpoint

focalpoint

a Convex mirror

Figure 8.2.2 (a) A convex mirror diverges rays. (b) A concave mirror focuses rays.


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The most familiar example of EM wave reflection is a plane bathroom mirror. The reflection staring back at you is called an image (Figure 8.2.3). The image is upright and the same size as the original object (you). It also appears to be the same distance from the mirror surface as the object but on the other side of the mirror (Figure 8.2.4).

Light rays reflected in all directions from the hand. Some of the rays strike the mirror and are reflected at an equal angle.

The direction of the rays reflected from the mirror surface appear to the people to have come from behind the mirror.

Two people looking at themselves in a plane mirror. The image of the two people behind the mirror.

Figure 8.2.4 Ray diagram illustrating reflection from a plane mirror. The diagram shows only a few representative light rays for simplicity.

A make-up or shaving (concave) mirror gives a magnified image. The nature of the image produced by a concave mirror depends on the location of the mirror’s focal point and the object. If you’re closer to the mirror than the focal length away, your image is upright and magnified. Move further away from the mirror than the focal length, your image is upside down (inverted). (See Figure 8.2.5.)

Figure 8.2.5 Reflection from a concave mirror. The boy’s reflection is inverted as he is outside the mirror’s focal length.

Describe one application of reflection for plane and concave surfaces.

Figure 8.2.3 Reflection from a plane mirror


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TRY THIS!SPOON MIRRORYou can observe your changing image by looking at the concave surface of a shiny metallic spoon. Hold the spoon at arm’s length and then slowly bring it close to your face. First you will see a reduced upside-down image of yourself, and then it will get bigger and bigger. When the spoon is nearly touching your nose, you should see an upright enlarged image of your nose—if you can still focus your eyes at this point! Flip the spoon over; now you have a convex surface. Is the image different or the same? Figure 8.2.6 Looking into the concave

surface of a spoon.

Concave mirrors have many applications. Car headlights and torches use concave mirrors to reflect light from the bulb forwards in a more concentrated beam. Radio telescopes have large parabolic dishes that collect faint radio signals from distant stars, galaxies and black holes; the radio waves reflect from the dish surface and focus onto a detector at the focal point (Figure 8.27). By scanning the sky, radio telescopes can form a detailed radio image of the distant cosmic object. Communications applications are described in detail in Section 8.4.

Figure 8.2.7 The Australia Telescope Compact Array comprises six parabolic dishes (five shown here), each 22 m in diameter. This radio telescope is located near Narrabri in New South Wales.


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A reflective convex surface will spread out or diverge incident rays. Convex mirrors produce images that are upright and reduced in size. They provide a wider field of view than a concave or plane mirror, so they are used extensively for road safety to provide greater visibility at blind intersections (see Figure 8.2.8) and as security mirrors in shops. Some side mirrors on cars are convex.

Figure 8.2.8 Roadside safety mirror on a coastal road. Mirrors like these enable car drivers to see round sharp bends (blind bends) in a road.

CHECKPOINT 8.21 Reflective surfaces can be planar, concave and convex in shape. Describe an application for each shape.2 Parallel rays strike plane, concave and convex mirrors. Compare the paths of the reflected rays, including a

diagram in your answer.

Describe one application of reflection for convex surfaces.


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8.3 EM wave refractionWhen an EM wave travels from one medium into another, some of the wave will be reflected at the interface and the rest will propagate through the new medium.

The speed of the transmitted wave depends on the medium and changes from one medium to another. If the wave is obliquely incident (strikes at an angle less than 90° to the normal) on a boundary interface, the wave bends. This is known as refraction.

Figure 8.3.1 shows wave fronts refracting across the boundary between an incident medium and a transmitting medium. You can see from the reduced spacing of the wave fronts that the wave has slowed down as it enters the transmitting medium. We can use the equation distance = speed × time and simple geometry to work out a mathematical law describing refraction. First, assume that the speed in the incident medium is v1 and that the speed in the transmitting medium is v2. Next we know that the wave takes the same time (Δt) to travel the distance BD in the incident medium and the distance AC in the transmitting medium (note the equal number of wave fronts). Since ΔABD and ΔACD are right-angle triangles, simple trigonometry gives:

ADBD

i

=sinθ

and ADAC

r

=sinθ

This simplifies to:BD

isinθ = AC

rsinθ

BD = v1Δt and AC = v2Δt, using distance = speed × time, so substitute these values to give: v v

i r

1 2

sin sinθ θ=

Rearrange to give:

v

vi

r

1

2

=sin

sin

θθ

This equation is referred to as the law of refraction or Snell’s law, which was named after Dutch astronomer and mathematician Willebrord van Roijen Snell (1591–1626) who proposed it in 1621. Snell’s law is more commonly expressed in terms of the refractive indices of the two media. In Section 8.1, the refractive index (n) of a medium was defined as the ratio of the speed of an EM wave in a vacuum (c) to that in the medium (v):

ncv

=

We can rewrite Snell’s law in terms of the refractive index, where ni is the refractive index of the incident medium, θi is the angle of incidence, nr is the refractive index of the transmitting medium and θr is the angle of refraction:

n sin θi = n sin θr

Incident medium

Transmitting medium

B

DA

C

v1Δt

v2Δtr

i

Define Snell’s law.

Figure 8.3.1 The refraction of waves across a boundary

REFRACTIVE LENSES

The physical concept of refraction has been employed

by humans to their advantage for thousands of years. Ancient Greeks and Romans made burning glasses (convex lenses) to start fires, and glass globes filled with water were used to magnify objects. The lens is a widely used optical device: it reshapes a wave front using refraction for a specific purpose. Point sources producing spherical wave fronts can be converted into beams of plane waves (such as overhead projectors), and parallel rays can be made to converge and form an image (such as cameras).


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Worked exampleQUESTIONA scuba diver shines a beam of light up towards the surface of the water. It strikes the air–water surface at 35° to the normal. The refractive index of the sea water is 1.38. At what angle will the beam of light emerge into the air?

SOLUTIONThe angle of incidence θi is 35°, the refractive index of the sea water is 1.38 and the refractive index of air is 1.00.

Snell’s law says:

ni sin θi = nr sin θr

Rearrange to make θr the subject of the equation:

θr = sinsin−1 nn

i i

r

θ

Substitute the values:

θr = sin. sin− ×1 1 38 35

1 = 52°

The light beam emerges from the water at an angle of 52° to the normal.

There are three possible outcomes for a refracted ray and they depend on the relative refractive indices of the two mediums and the angle of incidence.1 ni < nr In this case the wave is entering a denser medium and slows down. The

refracted ray will bend towards the normal (Figure 8.3.2).

air

normal

water

n r

n i

Figure 8.3.2 When ni < nr refracted ray bends towards the normal.

Solve problems and analyse information using Snell’s law.

TRY THIS!CREATING AN IMAGEAsk your teacher for a small glass or Perspex biconvex lens. Find a room in which you can easily see a tree from the window. Hold the lens up to the window and place a piece of white paper directly behind the lens to act as a screen. You will need to slowly move the paper screen away from the lens until you can see a sharp (not blurry) image of the tree on the piece of paper. The image of the tree will be smaller and upside down, it will be the same colour as the real tree and, if there is a breeze, you will even see the leaves of the tree’s image moving. The distance from the lens to the piece of paper when the image is sharp is the focal length of the lens. What do you think will happen to the tree image if you covered half of the lens with opaque cardboard? Try it and see.


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2 ni > nr

The wave enters a less dense medium and speeds up. The refracted ray will bend away from the normal (Figure 8.3.3).

air

normal

water

n r

n i

Figure 8.3.3 When ni > nr refracted ray bends away from the normal.

3 ni > nr and θi = critical angle

If the size of θi is increased, θr will also increase according to Snell’s law (ni sin θi = nr sin θr), as shown in Figure 8.3.4a and b. Eventually, the transmitted ray will be tangent to the boundary and θr will equal 90°.

The critical angle (θc) is defined as the special value of θi for which θr equals 90°. For incident ray angles greater than the critical angle, all of the wave’s energy will be reflected at the boundary back into the incident medium (Figure 8.3.4c and d). This is known as total internal reflection.

air

water

air

water

air

water

air

water

normal normalnormal

90°

normal

nr

r

i ii c i c r i

r

ni

nr

ni

nr

ni

nr

ni

a b c d

Figure 8.3.4 (a) and (b) As θi is increased, θr also increases. (c) and (d) When θi is equal to or greater than the critical angle θc, the incident ray is reflected at the boundary.

Identify conditions necessary for total internal reflection with reference to the critical angle.

Interactive

Module


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Worked exampleQUESTIONCalculate the critical angle for a diamond (refractive index 2.419) surrounded by air.

SOLUTIONThe critical angle will be a value of θi such that the angle of refraction is 90°.

ni = 2.419, nr = 1 and θr = 90°.

Snell’s law states: ni sin θi = nr sin θr

Rearrange to make θi the subject: θ θi

r r

i

nn

= −sinsin1

Substitute values: θi = sinsin.

− ×1 1 902 419

= 24°

Total internal reflection in optical fibresTotal internal reflection is the basis of the optical fibre (Figure 8.3.5). This technology is used to transfer large amounts of information in the form of light pulses. The optical fibre is a fine cylindrical tube made of glass or plastic. Its diameter can range from 1 µm to 1 mm, depending on the application. Light pulses travel along the fibre at close to the speed of light with only small losses in intensity. The light remains within the fibre as it strikes the sides of the fibre at values greater than the critical angle and is reflected thousands of times per metre (Figure 8.3.6).

Figure 8.3.5 A bent cylinder of jelly transmitting a red light beam. A red laser beam is being transmitted through and out (centre left) of the jelly by total internal reflection. The beam is able to travel the length of the mould even though it is bent, with negligible loss of intensity.


Outline how total internal reflection is used in optical fibres.

Activity 8.1

PRACTICAL EXPERIENCES

Activity Manual, Page 81

CHECKPOINT 8.31 Snell’s law states:

vv

i

r

1

2

= sinsin

.θθ

What do v1, v2, θi and θr represent?

2 Describe what happens when a light ray travelling through a glass block strikes a boundary between the glass and air in the following cases.a at an angle less than the critical angleb at an angle greater than the critical angle

3 Outline how a light ray could be made to travel along a glass optical fibre without any light leaking from the sides.

Figure 8.3.6 A bunch of optical fibres. The light is only visible at the ends of the fibres, not through the sides, because the light is reflected from the sides of the fibre and not transmitted.


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8.4 Communications technologies using EM waves

TelegraphThe first technology to utilise electromagnetism in Australia was the telegraph (Figure 8.4.1). Written messages were converted into Morse code—a series of short and long current pulses (‘dots’ and ‘dashes’)—and were sent along strands of iron wire.

PHYSICS FEATURELINKING AUSTRALIA TO THE REST OF THE WORLD

One-hundred and fifty years ago, before the telegraph was constructed linking Singapore to

Darwin, it took approximately 60 days for news to arrive in Sydney from London by ship. Imagine having to wait two months before you found out the result of a cricket game between England and Australia or to hear of the death of Princess Diana. In 1870 electrical engineer Charles Todd (1826–1910) led three teams to construct the overland telegraph from Darwin to Adelaide. More than 3000 km of cable was laid through some of the most inhospitable country and under very difficult conditions. In 1872, when the overland telegraph was completed and connected to the submarine (underwater) cable to Java, news from London was available within 48 hours.

Figure 8.4.1 Australia’s fragile link to the outside world. This photograph of Bob Carrew up a pole of the overland telegraph line was taken in 1921. Two strands of galvanised iron wire, like modern fencing wire, were mounted on wooden poles. The iron was not insulated, which meant the energy of the current pulses quickly diminished; therefore, the signal was retransmitted at repeater stations approximately 250 km apart.

1. The history of physics


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Telephone The next technological step was to transform a soundwave directly to

an oscillating electrical current so that a spoken message could be conducted along a telegraph line. A sound wave from spoken communication, a singer or a musical instrument is directed towards a microphone. In one kind of microphone (a ‘dynamic microphone’), the sound wave causes the delicate diaphragm to vibrate. A small wire coil attached to the diaphragm vibrates near a magnet, producing an oscillating current copy (or analogue) of the same shape as the original sound wave. This oscillating current is called an audio wave. These audio waves are amplified (have their intensity increased) and sent along metal wires to the receiver—a speaker that acts like a reverse microphone, converting the oscillating current back into a soundwave. The telegraph had become a telephone.

Many telephones still work like this today; however, metal (copper) telephone cabling is being replaced with optical fibre in Australia to increase the capacity of telephone lines and to take advantage of digital communication technologies. Many landline telephone calls therefore use both copper wire and optical fibre to connect callers. The transport of information along optical fibre does not use analogue EM audio waves; rather, it uses a digital stream of light pulses, where the signal is converted into a stream of numbers represented as a series of pulses. This will be described in more detail later.

RadioNow perhaps you are thinking that since the audio wave in a wire produces an EM wave, why can’t we get rid of all the wires and cables and transmit the EM wave through the air? This is what Italian physicist Guglielmo Marconi (1874–1937) was thinking when he began work on the first radio in the 1890s (Figure 8.4.2). Nevertheless, there are a few problems with this.

First, to transmit an EM wave you need an antenna (a metal rod connected to an electrical oscillator) whose size is of the order of the EM wavelength. An audio wave will have a frequency range that corresponds to the range of human hearing, which is 20–20 000 Hz, so the wavelength range of the audio wave will be 15–15 000 km. Now this is clearly a ridiculous size for an antenna! Second, if we were able to build such an enormous antenna and transmit an audio wave, we could only receive and listen to one signal. This is because all audio waves from different signals have the same frequency range. For example, if two audio waves were transmitted at the same time, say the sound of a news bulletin being read and the rock band Silverchair playing, you would hear both at the same time over your radio receiver—hardly a satisfying experience for the news junkie or rock enthusiast.

WIRELESS TECHNOLOGY

Italian physicist Guglielmo Marconi had developed his first

radio equipment by the age of 21; it had a range of 1.5 km. He made the first radio transmission across the English Channel in 1899 and the first transatlantic transmission in 1901. He shared the Nobel Prize for Physics with German physicist Karl Ferdinand Braun in 1909. Marconi later developed short-wave radio and established a global radiotelegraph network.

Figure 8.4.2 Guglielmo Marconi


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For wireless EM wave communication to work, the audio wave needs to be encoded onto a single high-frequency EM wave called a carrier wave. The carrier wave has a convenient wavelength for transmission via an antenna (say, 1–600 m), and each separate audio wave can be encoded onto its own separate frequency carrier wave. The user can select the audio wave they receive on their radio by tuning it to a selected carrier frequency (Figure 8.4.3). So supposing you’re in Newcastle and you want to listen to Silverchair, you could tune your radio to receive the carrier frequency 102.1 MHz (Triple J Newcastle); alternatively, if you want the news, you could tune to 1233 kHz (Local ABC Newcastle).

AM and FM modulation The process of encoding the audio wave onto a carrier wave is called

modulation: it is a kind of superposition, where waves of different frequencies are combined to form a single wave in such a way that the information contained in the audio wave is preserved. Two types of modulation are used in communications technology: amplitude modulation and frequency modulation.

Amplitude modulation (AM) is usually used for EM carrier waves with frequencies in the 535–1605 kHz range (referred to as the AM radio band). The carrier wave and the audio wave are combined in an electric circuit called a modulator; the carrier wave’s amplitude is varied so that the shape of the varying amplitude is a copy of the audio wave, thus preserving the features of the audio wave for later decoding (see Figure 8.4.4). The resultant AM wave is amplified and then transmitted from an antenna. Radio receivers contain a demodulating circuit that removes the carrier wave and sends the audio wave to the speaker.

Frequency modulation (FM) is commonly used for not only FM radio broadcasting with carrier waves in the 88–108 MHz range, but also mobile phone transmission where the carrier waves are microwaves with frequencies of 800 MHz to 3 GHz. The carrier wave and the audio wave are combined in an FM modulator circuit; in this case, the carrier’s frequency is varied in such a way that the pattern of the varying frequency reflects the shape of the audio wave (see Figure 8.4.4). The resultant FM wave will increase in frequency to indicate a peak in the audio wave and decrease to indicate a trough in the audio wave. Therefore, the features of the audio wave are preserved in the FM wave and can be recovered by a radio receiver.

carrierwave

audiowave

amplitudemodulatedwave

frequencymodulatedwave

Figure 8.4.4 Amplitude and frequency modulation

Outline how the modulation of amplitude or frequency of visible light, microwaves and/or radio waves can be used to transmit information.

Figure 8.4.3 To listen to your favourite radio station, turn the dial and select the appropriate carrier frequency.


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Electrical interference or ‘noise’, such as crackling and buzzing in a radio receiver, can be caused by electrical discharges or rapid variation in voltage from electric motors, light switches, lightning nearby, electric trains and so on. This kind of interference more closely resembles an AM than an FM signal because it involves rapid changes in amplitude; therefore, AM is more badly affected by such noise. For this reason, music broadcasts, where sound quality is important, use FM radio. Buildings, bridges, tunnels and car parks cause AM radio waves to be blocked or distorted, so FM radio is used more often in cities and built-up areas.

Although FM radio offers better reception and less interference, its broadcast range is less for the same power and it tends to be reflected by mountain ranges. In contrast, AM radio can be received over longer distances.

Light produced by lasers is used to carry information via optical fibre. This type of light has a small, fixed frequency range and so can be amplitude modulated by using an audio wave. However, it is more common to use light to carry digital signals, which are fundamentally different from analogue AM and FM waves. (This will be described later in this section.)

BandwidthSince the frequency of the wave shifts around in FM, both the radio transmitter and radio receiver must be able to access a small range of frequencies around the main frequency you’re tuned to. The size of this range of frequencies is called bandwidth. However, even in an AM radio signal there is a bandwidth. By definition, a sine wave of pure frequency must have constant amplitude, so an AM signal with varying amplitude is in reality a superposition of a small range of frequencies surrounding the frequency of the carrier you’re tuned to. The radio transmitter and radio receiver must be able to access this small range of frequencies, which is also called bandwidth.

In Australia, AM radio stations are separated by 9 kHz, so each AM broadcast has a bandwidth of 9 kHz. FM radio needs a much larger bandwidth. In Australia, FM radio stations are separated by 200 kHz, so fewer FM stations are available compared with AM stations. In general, the more information transmitted per second, the larger the bandwidth needed (which is why Internet connections with high data speed are called broadband).

TelevisionA television is a much more complicated piece of technology than a radio; however, television programs are transmitted in much the same way (Figure 8.4.5). A microphone is used to collect sound energy and convert it into EM energy (the audio wave); a camera collects light energy (the picture) and this is converted into EM energy (the video wave). The video and audio waves are combined with a carrier wave (with frequency in the range of 40 to 880 MHz) and then amplified for transmission to users. The audio component is frequency modulated and the video component is amplitude modulated. The television signal also contains information for colour, brightness and synchronising the audio and video components. Television broadcasts have a bandwidth of 7 MHz.

Analyse information to identify the waves involved in the transfer of energy that occurs during the use of television.

TRY THIS!MAKE YOURSELF INTO AN ANTENNA!Most modern cars now have keyless entry systems. This means you can remotely lock and unlock the car doors by using a small radio wave transmitter on your key ring. What is the maximum distance you can stand from the car and still unlock the doors? This distance is the range of the radio wave transmitter. Now press the metal key against your bare wrist or neck. This should increase the range of the radio wave transmitter as your body conducts the EM waves like an antenna. Does the range change if a taller or shorter person repeats the process?


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TV camera

microphone

audiowave

audiowave

videowave

videowave

synchronisingsignals

synchronisation

AMtransmitter

AMreceiver

FMreceiver

FMtransmitter

diplexer

transmittingantenna

receivingantenna

Radio waves (40–880 MHz)

television screen

loudspeaker

Figure 8.4.5 A television signal consists of two main parts: the video wave and the audio wave. The pictures and sound are synchronised prior to transmission. The two waves are separated by the television set: the audio wave is converted into sound, which comes out of the speakers, and the video wave is used to produce light patterns on the television screen.

Transmitting EM waves over long distancesThe next stage in our technological journey is to describe how it is possible to transmit EM waves over long distances so that radio, television and mobile phone signals can be received by large numbers of people. EM waves can travel long distances in straight lines through a vacuum with relatively small losses in energy. This is apparent when we use optical and radio telescopes to study EM waves from stars that are light-years away. However, problems arise when obstacles such as buildings, mountains and the curvature of the Earth get in the way. The solution is to use reflection to bounce the EM waves around obstacles. A number of clever solutions have been devised to make these reflections possible.

Earth’s ionosphere and radio wave reflectionEarly in the development of radio, it was discovered that high-frequency (HF) and very-high-frequency (VHF) radio waves in the 3–50 MHz range were reflected from a region of the Earth’s atmosphere called the ionosphere (Figure 8.4.6). The ionosphere extends from 50 to 500 km above the surface of the Earth; the gas molecules in this region are ionised by EM radiation from the Sun. The degree to which the ionosphere reflects radio waves depends on the time of day (less ionisation of gas molecules occurs at night), the season and solar activity (such as sunspots and solar flares—see pages 307–309 in Chapter 16 for more information).

Describe one application of reflection for radio waves being reflected by the ionosphere.


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weather balloon

Mt Everest

250 km

200 km

150 km

100 km

50 km

0 km

ionised F layer

ionised E layer

ionised D layer

shuttlereflectedshort-waveradio signals

northern lights

rocket

meteorites

spy plane

jet clouds

Figure 8.4.6 The ionosphere has three distinct layers called D, E and F layers. The D layer absorbs radio waves; solar flare activity greatly increases the ionisation of the D layer, which can severely affect radio communications. The E and F layers reflect HF and VHF radio waves. The F layer is the most important as its high altitude allows the longest communication paths; also, it reflects the highest frequency radio waves.

HF and VHF radio waves can travel thousands of kilometres around Earth’s curvature by bouncing off the ionosphere (Figure 8.4.7). These frequencies are used for long-distance radio communications by defence, maritime, aviation and emergency services and remote broadcasters. The reflective properties of the ionosphere are constantly monitored by the Australian Government’s Ionosphere Prediction Service. They advise HF and VHF radio users of the highest frequency that the ionosphere will reflect.


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Earth

ionosphere

transmitter

receiver

Figure 8.4.7 Radio waves travel long distances in straight lines (shown in purple), but eventually the curvature of the Earth gets in the way. HF and VHF radio waves (shown as blue and red, respectively) are reflected from the ionosphere. Radio waves greater than 50 MHz (shown in green) pass straight through the ionosphere into space.

Geosynchronous satellitesThe ionosphere is transparent to radio waves used for television broadcasting (40 to 800 MHz) and microwaves used in mobile phones (800 MHz to 3 GHz).

This means that these higher frequency EM waves pass straight through the atmosphere and into space. In order for these EM waves to travel long distances to reach people all over Australia, the waves are reflected back to the Earth’s surface by geosynchronous satellites (Figure 8.4.8).

A geosynchronous satellite orbits at an altitude of 35 580 km directly above the equator. Its orbital period is exactly one Earth day, so it stays above the same spot on Earth at all times. The satellite collects the transmitted EM waves using a parabolic dish. This dish acts like a concave mirror and focuses the incident waves to a central receiver. The collected wave is then amplified and retransmitted by the satellite back towards the Earth. More parabolic receiver dishes on Earth within the satellite’s range or ‘footprint’ collect the EM waves so that the information can be accessed by distant users, such as international telephone calls or satellite TV.

Describe ways in which applications of reflection of light, microwaves and radio waves have assisted information transfer.

SCIENCE OR SCIENCE FICTION?

Arthur C. Clarke (1917–2008) made a living from imagining and speculating on technology of the future like other science-fiction authors.

His most famous novel 2001: A Space Odyssey, which was published in 1968, includes futuristic technologies, such as robotics, artificial intelligence and interplanetary space travel. Clarke served with the RAF during the Second World War and worked with the emerging radar technology; he later obtained honours degrees in physics and mathematics. In 1945 he wrote an article for the radio and electronics magazine Wireless World, entitled ‘Extra-terrestrial relays—can rocket stations give worldwide radio coverage?’, outlining a way of using rocket technology to facilitate long-distance communications. This article was a successful prediction of the geosynchronous satellite technology that was to emerge 20 years later.

geostationary satellite

groundstation ground

station

Earth

Figure 8.4.8 Using geosynchronous satellites to reflect EM waves over large distances


156

As the EM wave has travelled in excess of 70 000 km from the transmitter, to the satellite and then to the receiver, there is a delay of about one-quarter of a second; for telephone conversations, this can be quite annoying.

Limitations of the electromagnetic spectrum in communicationsThe number of frequencies in the electromagnetic spectrum that can be used for communication purposes is limited. AM radio stations are spaced every 9 kHZ; FM stations, every 200 kHz; and television stations, every 7 MHz. Existing stations find they require additional bandwidth to provide features like stereo, surround sound and high-picture definition. Thus there is a limit not only to the number of radio and television stations that can operate, but also to the amount and quality of information they can broadcast.

The proliferation of mobile phone technology has added more pressure. Initially, mobile phones were assigned the spare microwave spectrum above radio and television but below weather and military satellites; however, now there is greater demand for bandwidth as the number of users increases and as mobile phones become more sophisticated by incorporating pictures and wireless Internet access.

The most desired frequencies for communications purposes (100 MHz to 3 GHz) are in high demand. Bandwidth is auctioned to media and telecommunications companies and allocated to some public organisations, such as the Australian Broadcasting Corporation (ABC).

Two main problems result from limited bandwidth. First, the high cost of communications technology combined with the restricted access to bandwidth means that communications services can be expensive for users, especially in areas where the density of users is low. Second, emerging technologies struggle to compete with existing technologies in accessing limited bandwidth.

Digital technologyAt the moment, the apparent solution to our limited bandwidth problems is the digital revolution. You may have noticed in the past ten years that there has been increasing use of digital technologies as analogue services for landline phones, mobile phones, television and radio are being phased out. Digital technologies allow large amounts of information to be transferred faster using less bandwidth, information in digital form is relatively unaffected by noise and interference, and it is processed more quickly by computers. For example, digital technology will allow a current television station to transmit four times more information than current analogue levels that use the same bandwidth—that’s four digital channels in the space needed for one analogue channel. Sounds fantastic, doesn’t it? But what is digital technology?

Digital technology handles information like sound, text and pictures that has been converted into binary code—a number system that is base 2: it only has two numbers, 0 and 1 (Figure 8.4.9). Information in binary or digital form is very different from analogue information: digital information is a long series of 0s and 1s and the signal is a series of discrete on/off pulses; however, analogue information, such as audio and video waves, is in continuous wave form.

An electronic device called an analogue-to-digital converter is used to change analogue waves into digital signals; for example, in your mobile phone. The analogue-to-digital converter periodically samples the audio or video wave, measures the voltage and stores it as a binary number. The audio or video wave becomes a series of binary numbers listed in sampling order.

Discuss problems produced by the limited range of the electromagnetic spectrum available for communication purposes.

0

1

2

3

4

5

00000

00001

00010

00011

00100

00101

Decimal(base 10)

Binary(base 2)

Digital signal

Figure 8.4.9 Binary code and the corresponding digital signal


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The sampling process does not make an exact copy of the analogue wave; the voltage at every point in time is not measured. For example, the sampling rate for the audio wave from a mobile phone handset is 8000 times a second. This produces a digital signal with enough of the original information for the speaker’s voice to be recognised by the listener. Higher sampling rates are needed for video waves for a high-quality picture. When a digital signal arrives at a receiver, a digital-to-analogue converter is used to reconstruct the original analogue audio or video wave from the binary code. This is what a digital set-top box does with the digital television signal received by the household television aerial (Figure 8.4.10).

Digital signals can be transmitted by combining them with carrier waves just like analogue waves; these carrier waves are modulated by the digital signal. Digital signals are commonly transmitted as light pulses along optical fibres. A high-energy laser with a small frequency range is used to produce on/off pulses of light that correspond directly to the binary code.

Almost every aspect of the communications industry is undergoing a digital revolution. Analogue mobile phones are now part of history and analogue television will be phased out in Australia by 2013. Radio, television, landline telephones, mobile phones, broadband and wireless Internet all use data in digital form. In addition, many devices store data in digital form; for example, compact discs (CDs), digital video discs (DVDs), computer hard drives, USB drives, digital cameras and MP3 players.

CHECKPOINT 8.41 Compare amplitude modulation and frequency modulation. Include a diagram in your answer.2 Why is reflection from the ionosphere useful for high-frequency radio wave transmission?3 Describe how a satellite is used to increase the distance range of microwave and television radio wave

transmissions.4 Outline two problems that arise from the limited range of the electromagnetic spectrum.5 Identify four types of digital communication data.

Identify types of communication data that are stored or transmitted in digital form.

Activity 8.2

PRACTICAL EXPERIENCES

Activity Manual, Page 85

MODULATING DIGITAL

Three different types of modulation are used with

digital signals:1 Amplitude-shift keying (ASK):

The 0s and 1s are impressed into the carrier wave by shifting abruptly between high and low amplitudes. This process is similar to amplitude modulation with analogue waves.

2 Frequency-shift keying (FSK): The 0s and 1s are represented by shifts between two frequencies in the carrier wave.

3 Phase-shift keying (PSK): The amplitude and frequency of the carrier wave remain constant, and the 0s and 1s are represented by shifts in the phase of the carrier wave.

a Analogue to digital b Digital to analogue

Audio wave (from microphone)

Amplitude read

Amplitude read (sentto the loudspeaker)

76543210

Volt

age

76543210

Volt

age

Time

Time

76543210

Volt

age

TimeVoltage readings at44 100 times a second

Digital signal transmitted

Digital signal convertedback into analogue

01356777654322234

Voltage converted into digital signals

Figure 8.4.10 (a) Converting an analogue audio wave into a digital signal. (b) Converting a digital signal into an audio wave.

8 PRACTICAL EXPERIENCES

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CHAPTER 8This is a starting point to get you thinking about the mandatory practical experiences outlined in the syllabus. For detailed instructions and advice, use in2 Physics @ Preliminary Activity Manual.

ACTIVITY 8.1: RELATIONSHIP BETWEEN ANGLES OF INCIDENCE AND REFRACTIONUse a light box to produce a narrow beam of light. Direct the light beam into a Perspex block, trace the rays and measure the angles of incidence and refraction. Make measurements for a number of different angles of incidence and then graph your data.Equipment list: transformer, light box, rectangular block of Perspex, blank paper, ruler, pencil, protractor, graph paper, calculator.

Discussion questions1 What physical law are you using to interpret your results in this

investigation?2 Explain why it is important to make measurements for a number of

different angles of incidence.3 Are there any data points that you should exclude from your graph?

Justify your decision to exclude these points.4 Predict how the slope of the graph would change if you repeated the

experiment, this time placing the Perspex block in a tank of water so that the initial medium was water instead of air.

light box

Perspex blocks

Figure 8.5.1 A light box and Perspex blocks

Perform an investigation and gather information to graph the angle of incidence and refraction for light encountering a medium change, showing the relationship between these angles.

Perform a first-hand investigation and gather information to calculate the refractive index of glass or Perspex.


159

ACTIVITY 8.2: RESEARCH AND DISCUSS THE PHYSICAL PRINCIPLES UNDERLYING COMMUNICATIONS TECHNOLOGYUse the research template to gather appropriate information on one of the suggested communication technologies.

Research templateA partial template with sample entries and references is shown (the complete template and a list of references for each application are provided in the in2 Physics @ Preliminary Activity Manual ).

APPLICATION (CIRCLE):

GLOBAL POSITIONING SYSTEM

CD TECHNOLOGY THE INTERNET DVD TECHNOLOGY

UNDERLYING PHYSICAL PRINCIPLES OF THE APPLICATION

NOTES AND DIAGRAMS REFERENCES AND SOURCES

Identify the information output of the application, i.e. what does the user receive (e.g. music, pictures, text)?

• Output is either text or picture.

• The GPS receiver gives the user a position coordinate.

• This is given relative to reference locations in the receiver software (e.g. displayed on a street map or given as a latitude and longitude).

Howstuffworks, Marshall Brain and Tom Harris

http://electronics.howstuffworks.com/gps.htm

Date accessed: 18 March 2008.

Identify the types of waves that are used in this application(e.g. sound, light, radio waves, microwaves).

• Satellites transmit two microwave carrier signals.

• The L1 frequency (1575.42 MHz) carries the navigation message.

• The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay.

The Global Positioning System, Peter H. Dana 1994

http://www.colorado.edu/geography/gcraft/notes/gps/gps

–f.html

Date accessed: 18 March 2008.

Process the information you have gathered and give a short oral presentation to your class, discussing the physical principles underlying your chosen technology.

Identify data sources, gather, process and present information from secondary sources to identify areas of current research and use available evidence to discuss some of the underlying physical principles used in one application of physics related to waves, such as:• global positioning system• CD technology• the Internet• DVD technology.

8 Communication applicationsof EM wavesCoCoCoCoCommmmmmmmmmunununununicicicicicatatatatatioioioioion n n nn apapapapapplplplplplicicicicicatatatatatioioioioionsnsnsnsnsofofofofof E E E EEM M MM M wawawawawavevevevevesssss

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• An oscillating charged particle produces an oscillating electric field and an oscillating magnetic field.

• Together, oscillating electric and magnetic fields are called electromagnetic (EM) waves.

• EM waves are transverse waves.• EM waves propagate in most media and in a vacuum.• The speed of EM waves in a vacuum (c) is

300 000 km s–1 (3 × 108 m s–1).• The ratio of the speed of an EM wave in a vacuum (c)

to that in matter (v) is known as the refractive index (n).• The many possible variations in frequency produce

a spectrum of EM waves called the electromagnetic spectrum.

• Radio waves, microwaves, infra-red (IR), visible light, ultraviolet (UV), X-rays and gamma rays are all different types of EM waves.

• The Earth’s atmosphere absorbs some IR, and nearly all the UV, X-rays and gamma rays that are emitted by the Sun.

• A reflective concave surface will tend to focus waves.• A reflective convex surface will tend to diverge waves.• The speed of a wave changes as it moves from one

medium to another. This is called refraction.• The refraction of EM waves can be described using

Snell’s law:

v

vi

r

1

2

=sin

sin

θθ

• When the angle of incidence is greater than the critical angle, the incident ray undergoes total internal reflection.

• Total internal reflection is the basis of the optical fibre.• A sound wave can be converted into an EM wave called

an audio wave.• An audio wave is a copy, or analogue, of the shape of

the original sound wave.• Audio waves are encoded onto a single high-frequency

EM wave called a carrier wave in a process called modulation.

• There are two types of modulation: amplitude modulation (AM) and frequency modulation (FM).

• In AM the carrier wave’s amplitude is varied so that the shape of the varying amplitude is a copy of the audio wave.

• In FM the carrier wave’s frequency is varied in such a way that the pattern of the varying frequency reflects the shape of the audio wave.

• Both AM and FM signals consist of a range of frequencies that transmitters and receivers need to access. This range of frequencies is called bandwidth.

• A television transmission consists of an audio wave and a video wave encoded onto a carrier wave.

• Reflection is used to bounce the EM waves around obstacles such as buildings, mountains and the curvature of the Earth.

• High-frequency (HF) and very-high-frequency (VHF) radio waves can travel thousands of kilometres around Earth’s curvature by bouncing off the ionosphere.

• A geosynchronous satellite is used to reflect microwaves used for international telephone calls and satellite television over large distances.

• The number of EM wave frequencies available for communications purposes is limited.

• Bandwidth requirements mean there is a limit to not only the number of radio and television stations that can operate, but also the amount and quality of information they can broadcast.

• Digital technologies allow large amounts of information to be transferred faster using less bandwidth.

• Digital technology handles information, like sound, text and pictures, that has been converted into binary code.

• Radio, television, landline telephones, mobile phones, broadband and wireless Internet all use data in digital form.

• Compact discs (CDs), digital video discs (DVDs), computer hard drives, USB drives, digital cameras and MP3 players all store data in digital form.

Chapter summary


161

PHYSICALLY SPEAKINGThe jigsaw activity below will help you identify and summarise the key ideas in this chapter. Follow the steps below and you will end up with a point-form summary of the entire chapter.

1 Your teacher will organise the class into home groups of four people. Each person in the home group will be given a different section of this chapter to work on.

2 Spend ten minutes on your own, reading over your allocated section.

3 Write what you think are the five most important concepts (in point form) in your allocated section.

4 Join with other people in your class who have the same allocated section to form an expert group.

5 Share your five important concepts with the other members of the expert group. You will find others in the group may have different concepts from your own.

6 Discuss your ideas and then as a group, if necessary, pick the five most important concepts from those suggested.

7 As a group, write a summary of the five most important concepts.

8 Return to your home group and outline your five important concepts to the others. Offer other home group members a copy of your summary. You will now have four summaries that will cover all of Chapter 8.

REVIEWING 1 Explain why electromagnetic waves are transverse rather than longitudinal

waves.

2 If there are no particles in an absolute vacuum, what is it that oscillates as an electromagnetic wave passes through?

3 Gamma rays, IR, microwaves, radio waves, UV, visible light and X-rays are types of electromagnetic waves.a Arrange the electromagnetic wave types in order from lowest frequency

to highest frequency.b Which wave type has the shortest wavelength?

4 Calculate the speed of an IR wave travelling through a beaker of liquid benzene (benzene has a refractive index of 1.50).

5 Red light produced by a laser has a frequency of 4.08 × 1014 Hz. Calculate the wavelength of this red light travelling through a vacuum.

6 A household microwave oven produces a frequency rating of 2450 MHz. Calculate the wavelength of the microwaves produced by the oven.

7 If it takes 8 minutes for electromagnetic radiation from the Sun to reach the Earth’s surface, how far away is the Sun from Earth?

8 What is the angle of reflection for a ray incident normally (at 90°) on a smooth surface?

Solve problems and analyse information by applying the mathematical model of v = fλ to a range of situations.

Review questions


162

9 Plane wave fronts are incident on three mirror shapes, as shown in Figure 8.5.2. Copy the diagrams and draw in the reflected wave fronts for each mirror.

planemirror

concavemirror

convexmirror

Figure 8.5.2

10 What is the angle of refraction for a beam striking an air–water boundary perpendicularly?

11 When a beam of light travelling in air enters a glass block, what happens to the speed of the light? Describe what happens to the light’s frequency and wavelength as it enters the glass block (use a wave front diagram in your answer).

12 What is an audio wave? Outline how one is produced.

13 Define the term bandwidth.

14 List three types of energy involved in receiving a television broadcast on your home television.

15 Identify two factors that can vary the reflective properties of the ionosphere.

16 Compare an analogue signal with a digital signal. Include a diagram in your answer.

17 Complete the table below to summarise the electromagnetic spectrum range used in modern communication technologies. The first row has been completed for you.

COMMUNICATION TECHNOLOGY EM WAVE TYPE USED TO CARRY DATA APPROXIMATE BANDWIDTHAM radio Radio waves (535–1605 kHz) 9 kHz

FM radio

Television

Mobile telephone

Internet

Analyse information to identify the electromagnetic spectrum range utilised in modern communications technologies.


163

SOLVING PROBLEMS 18 Calculate the angle of refraction for a ray incident in air at 30° on a block

of crown glass (refractive index 1.52).

19 A laser beam strikes an air–liquid surface at an angle of 55° to the normal. The refracted ray is observed to be transmitted into the liquid at 40°. What is the refractive index of the liquid?

20 Calculate the critical angle for light travelling through water into air. The refractive index of water is 1.33.

21 A glass block with a refractive index of 1.55 is covered with a layer of water of refractive index 1.33. For light travelling from the glass to water, what is the critical angle at the glass–water interface?

22 Using a block of transparent unknown material, it is found that a beam of light travelling inside the material is totally internally reflected at the air–block interface at an angle of 48°. What is the block’s refractive index?

EXTENSION 23 Light of wavelength 600 nm in a vacuum enters a block of glass with a

refractive index of 1.5. a Calculate the wavelength inside the glass. b What colour would it appear to be to someone embedded in the glass?

(You will need to consult a table of colour frequencies.)

24 Prove to someone looking straight down into a swimming pool that any object in the water will appear to be at three-quarters of its true depth.



164

PHYSICS FOCUSNEW PLASTIC TECHNOLOGY SET TO REVOLUTIONISE FIBRE OPTICS8/11/05 – Australian researchers have developed optical fibre made of plastic rather than glass—a technique which is set to revolutionise the use and manufacture of the technology around the globe.

Traditionally made of glass or silica, optical fibres are expensive to produce, fragile and not very flexible—which limits their application.

But three researchers, based at the University of Sydney’s Optical Fibre Technology Centre (OFTC),

found a way to make polymer optical fibres that can perform competitively with silica while being far easier and cheaper to make.

Dr Martijn van Eijkelenborg, Dr Maryanne Large and PhD student Alexander Argyros on Monday received this year’s Australasian Science Prize for their work over the past five years.

P5. Describe the scientific principles employed in particular areas of research

Figure 8.5.3 Australian scientists Maryanne Large, Martijn van Eijkelenborg and Alexander Argyros


165

‘What we have done is to change the material (of the optical fibres),’ Dr Large said on Monday.

‘I think what’s really significant about our work is we’ve actually found a kind of cheap way of making this gourmet fibre and mass producing it.’

Optical fibres are thin rods of glass which reflect and carry light and are wrapped in a low-density plastic and cabling.

They are used widely in communications, both in computer networks as a fast Internet connection source and in telecommunications.

Also used as an instrument in microsurgery, they can project images from inside the body and help surgeons see in hard-to-reach places.

Making plastic fibre optics was no easy task due to the material’s lack of transparency and reflection rate.

It also had a higher light absorption index than glass.But the team used a microstructured pattern

around an air core to overcome the problem.A pattern of concentric rings around the core

reflects light of particular frequencies back so that it cannot escape the core.

As the light travels through air rather than the polymer, the problem of the transparency of the polymer is overcome.

The University of Sydney team did not originate the idea of using microstructured fibres to guide light, but they were the first to use it on plastic.

Dr Large said she hoped to see the polymer fibre optics commercialised in the next few years.

‘We have had very serious interest from a number of major companies, actually, so I would certainly hope in the timescale of a few years we would have something commercialised,’ she said.

The Australasian Science Prize, first established in 2000, is awarded by the Australasian Science magazine, which is published monthly, to recognise outstanding research by an individual or small group.

Source: AAP NewsWire, 8 November 2005,www.industrysearch.com.au/news/viewrecord.aspx?ID=18637accessed 1 March 2008.

1 Optical fibres utilise the wave property of refraction. Define refraction.

2 Describe the concept of total internal reflection. Under what circumstances does it occur?

3 Outline how an optical fibre is used to transmit light. Include a diagram in your answer.

4 Compare the structure of the glass fibre with the plastic fibre using the information in the article.

5 What medium does the light travel through in the plastic fibre?

6 Describe one use of optical fibres in communications technology.

7 Explain why a flexible plastic fibre is preferable to a fragile glass fibre.

8 Assess the impact the new plastic fibre may have on communications technologies when it is commercialised.

EXTENSION 9 Contrast the waves used to communicate by mobile

phones and landlines.10 Justify the following statement: ‘Wireless

communications that use EM waves have revolutionised the way we communicate.’

166

Multiple choice(1 mark each) 1 Which of the following groups of waves are classified

as mechanical waves?A surface water waves, soundB sound, slinky, infra-redC light, slinky, rope flickedD radio, surface water waves, sound

2 Two pulses of the same amplitude were sent down a piece of rope towards each other (see Figure 8.6.1).

v = 0.5 m s–1 v = 0.6 m s–1

2.0 m

Figure 8.6.1 Two pulses on a rope

What will the resultant wave look like 1 s from now?

A B

C D

3 Which sections of the EM spectrum are least absorbed by the Earth’s atmosphere?A infra-red, gamma, radioB gamma, radio, visibleC X-ray, radio, infra-redD radio, visible, infra-red

4 Which of the following pairs use a digital format?A videotapes and CDsB MP3 files and DVDsC computer hard drives and 3½ inch floppy discsD USB drives and audiotapes

5 A light ray travels through the air and strikes a glass prism at 30°. The angle of refraction is 19°. What is the speed of light in the glass block?A 1.52 m s–1

B 0.65 m s–1

C 4.6 × 108 m s–1

D 2.0 × 108 m s–1

Short response 6 A red laser produced light with a frequency of

4.28 × 1014 Hz. Calculate the wavelength of this red light. (2 marks)

7 A fish-finding sonograph set to 150 Hz detects the seabed 5.4 m below the boat hull. The sound pulse is emitted by the sonograph and then received 7.2 ms later. Calculate the speed of sound in the salt water below the boat. (3 marks)

8 As part of a study, a national parks officer has attached identical radio collar transmitters to two Tasmanian devils. The radio signals are detected using an aerial. The radio signal received from one collar is three times stronger than the other. Estimate the relative distances of the two Tasmanian devils from the aerial. (3 marks)

The review contains questions in a similar style and proportion to the HSC Physics examination. Marks are allocated to each question up to a total of 25 marks. It should take you approximately 45 minutes to complete this review.

2


167

Extended response 9 During the course of your studies, you conducted a

first-hand investigation to determine the refractive index of glass or Perspex.a Briefly outline the procedure you used in this

investigation. (2 marks)b Below is a set of data collected by a student

during class to determine the refractive index of water. Use it to produce a graph to determine the refractive index. (3 marks)

ANGLE OF INCIDENCE (θi ) ANGLE OF REFLECTION (θr ) 0.00 0.00

5.00 3.76

10.00 7.50

15.00 11.20

20.00 14.90

25.00 18.50

30.00 22.10

35.00 25.50

40.00 28.90

45.00 32.10

50.00 35.20

55.00 38.00

60.00 40.60

65.00 43.00

70.00 45.00

75.00 46.60

80.00 47.80

85.00 48.50

c How would this graph change if the data were collected for Perspex, given that the refractive index of Perspex is 1.4? (1 mark)

10 a Compare and contrast the effectiveness of optic fibres in communications to copper wiring. (2 marks)

b The following is an extract of a transcript with Australian physicist Professor Louis Davies (1923–2001), an early researcher in optic fibres. Briefly discuss the contributions this research has made to society. (4 marks)

The research lab continued to be responsible for the semiconductor physics work which I had

brought with me and for the optical fibre work which by then had started in the company, but it also did quite a lot of work in electronics, telecommunications and defence communications. Optical fibre became a substantial part of the work. We started with hollow optical fibres filled up with dry-cleaning fluid—saturated hydrocarbons—which Graeme Ogilvie, a scientist in the CSIRO Tribophysics Division, had worked out would not absorb much light. So, if one made hollow tubes—kilometres long, taking a long while to fill from one end with liquid—those fibres would be of considerably lower transmission loss than the current versions of optical fibres with their solid cores. We made an experimental telecommunications system in Australia, setting it up at the Australian National University in Canberra because of the laws relating to access to communication in the public domain across roadways and so forth. We rapidly learnt one important aspect of liquid-filled optical fibres: unless both ends are at the same height, the liquid fairly rapidly drains out—in spite of the difficulty of getting it in there! Anyway, that was in a sense a minor exercise.

We then got into the business of developing and making optical fibres with solid cores. Being the only facility in Australia which could do it, we did quite a lot of defence and general commercial work. Perhaps one mistake was that, as a company, we didn’t move into cabling the optical fibres. No-one who was in telecommunications really wanted to buy fibres, they wanted to buy cables containing fibres. Ultimately AWA, Metal Manufactures and an American company, Corning, formed a company called Optical Wave Guides (Australia). Later, when I was a director of AWA, we sold our interests in that—primarily the equipment and know-how that we had developed in the lab—for about $13 million. That made me feel quite comfortable with the previous work of the laboratory.

Craig, D. (1999) Interviews with Australian Scientists: www.science.org.au/scientists/ld.htm#fibres, Australian Academy of Science.

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