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physipics

for students of Physics, aged 13-16

and for their teachers

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

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

Waves spread out from sources.

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.

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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.

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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.

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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.

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To tell the truth …

2 = 27 - 25

2 = 8 x 0.25

2 = 100 ÷ 50

2 + 0 = 2

… only zero can go here

2 + 27 = 2 + 27

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

… the possibilities are endless.

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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

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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.

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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.

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Extra letters

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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

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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.