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.
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.
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
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.
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
There are many correct links.
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
Page 75
Page 79
Page 81
Page 83
Page 89
Page 91
Try these … but not in this order …
analogue
diffraction
eardrum
emission
energy
epicentre
equation
evidence
gradual
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
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