physipics

for students of Physics, aged 13-16

and for their teachers

TM

WAVES review and revision book: hints and answers

but mostly just hints … It’s YOUR brain that matters here … we’ve left some thinking for you to do

Page 9

A wave is a repeating motion that transfers energy, and information, without transfer of matter.

Wavelength provides one way to measure waves. Amplitude provides another way.

The trough of a water wave is a low point. The wave crest is a high point.

Page 13

A period of time could be measured in centuries … seconds, milliseconds.

The period of a water wave is usually measured in seconds.

All waves have wavelength and wave speed. A wavefront is a line drawn along the crest or the trough of a wave.

We use ideas about water waves to help us to understand other kinds of waves, such as sound waves and light waves. So we use ‘wavefronts’ to understand those, too.

Page 17

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Page 25

Sound waves have compressions and rarefactions of particles of the medium.

Each particle vibrates, but the wave moves forwards.

Sound travels as longitudinal waves. The vibration of the medium is parallel to the direction of travel of the wave.

Water surface waves are transverse waves. The vibration of the water surface is perpendicular to the direction of travel of the wave.

The relationship between frequency and wavelength is an inverse one, for waves that have the same speed.

There is also an inverse relationship between frequency and period of waves.

Page 29

Incoming sound waves make eardrums vibrate. Small bones transmit the vibrations onwards. In the cochlea, vibrations become electrochemical signals.

Sounds that have frequency higher than about 20 kilohertz are ultrasound. If the frequency is less than about 20 hertz then they are infrasound.

Systems with amplifiers increase the amplitude, and the loudness, of sounds.

Page 33

To tell the truth …

2  =  27 - 25

2  =  8 x 0.25

2  =  100 ÷ 50

… only zero can go here

2 + 0  =  2

2 + 27  =  2 + 27

… anything can go here and here … just as long as they are both the same

… the possibilities are endless.

Page 35

3 x 1  =  3

… only one can go here

3 x 27  =  3 x 27

… anything can go here and here … just as long as they are both the same

Page 39

A seismometer measures vibrations of the Earth, and seismic means ‘to do with Earth vibrations such as from Earthquakes’. The Earth acts as a medium for waves to travel through.

Waves spread through the Earth from the epicentre of an earthquake in different ways. S-waves are transverse – they can travel through solids but not through liquids. P-waves are longitudinal.

The waves can experience gradual refraction as they travel through material of gradually changing density. The speeds of the waves change, and they gradually change direction.

There can be abrupt, or sudden, refraction at boundaries. P-waves experience abrupt refraction as they cross the boundary with Earth’s core.

Refraction, as well as the inability of S-waves to pass through the core, produce large shadow zones where little or no vibrations are detected.

High amplitude seismic waves can be very destructive.

Page 43

Water-surface waves, sound waves, and light waves can all be:

•  reflected at surfaces

•  refracted at boundaries

•  diffracted at gaps and obstacles

The medium for transmission of water surface waves is, of course, the surface of water.

Sounds can be transmitted by solids, liquids and gasses, but not by a vacuum.

Light waves can travel through a vacuum, and also through some materials. Air, for example transmits visible light, but a solid wall does not. However, glass is a solid medium that can transmit visible light.

Absorption of waves involves taking energy from waves, partly or completely.

A floating object can obtain energy from a water wave. Solids, liquids and gases can absorb sound waves.

Some solids, liquids and gases absorb energy quickly from light waves, but many, such as glass, absorb energy slowly. Light can travel for long distances through glass.

Page 49

Page 55

Extra letters

Page 61

To tell the truth …

always true

3  =  3

Lorem ipsum dolor sit amet, consectetur adipiscing elit.

always ‘not equal’

4  ≠  4 + 1

4  =  4 x b

only true IF  b = 1

always true

3 + b  =  3 + b

5 x b  =  5 x c

only true IF  b = c

Page 65

A human body transmits radio waves well, with very little absorption.

However, the body is good at absorbing infrared radiation, and its energy has a warming effect.

Skin absorbs some visible radiation, but mostly it reflects it.

Absorption of ultraviolet radiation causes sunburn, and it can lead to skin cancer.

Some UV radiation, as well as x-rays and gamma rays, can ionise atoms. In the human body that can cause harmful chemical changes. But low doses are not seriously harmful, and so x-rays and gamma rays are useful for medical body imaging.

X-rays and gamma rays can kill cells. In radiotherapy they are used to kill cancer cells, and so to cure cancer.

Page 67

Page 71

The purpose here is to learn by discussion of ‘justified’ links.

Discuss with several people if possible. Check what your teacher says.

You’ll remember more easily than by just looking at ‘answers’.

Page 73

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Page 81

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Page 89

Page 91

Try these … but not in this order …

analogue

diffraction

eardrum

emission

energy

epicentre

equation

evidence

infrasound

lens

light

longitdunal

signal

skin

transverse

wifi

yellow

zone

loudness

megahertz

nanometre

non-ionising

not equal

retina

Page 92

Extra letters

Page 93

Page 93 presents an old favourite word game with a new twist.

Play in pairs. Use ‘waves’ words. Make sure you know what they mean.

Page 94

Some ideas, to get started.

emission

metre

infrared

frequency

X-ray

principal focus

angle of incidence

Page 95

Some ‘opposites’:

S-wave

object

upright

optical power

compression

P-wave

image

inverted

focal length

rarefaction