Previously
From Page 288 Sound waves travel through the air from a
vibrating source.
From Page 294 Light can travel through empty space.
Unit 14
Waves
Why are waves so
important?
We can use the idea of waves to explain
how light, sound and many other
phenomena work.
For example, the science of waves can
help us to understand how we can use
ultrasound in physiotherapy. A patient
with arthritis can be helped by directing
ultrasound waves into their joints. The
waves are absorbed, transferring energy to
the tissues and speeding up healing.
To understand what waves are and how
they behave, we can study waves on water.
In this Unit, you will learn:
how water waves behave;
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how sound and light waves differ;
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how waves transfer energy.
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Key words
crest
disturbance
echo
energy
frequency
information
longitudinal wave
receiver
ripple
ripple tank
source
speed
transverse wave
trough
wave
wavelength
PHYSICS 305
Unit 14
Where do waves come from?
When we think of waves, we often
think of waves on the sea. The
wind blows over the sea, and this
causes waves. Waves are bigger
after a big storm.
The waves travel across the surface of
the sea. Their speed is a few metres
per second. Some sea waves may travel
thousands of kilometres, but eventually
they hit the land where they break.
This surfer is pushed along by a breaking wave.
A tsunami is a giant wave that can produce great damage. Tsunamis are not
caused by the wind.
Find out the three different causes of tsunamis.
How does water move to make a wave?
If you sit in a small boat adrift in the middle of the sea, you may notice something
interesting. The waves make the boat go up and down, but the boat doesn’t move along
with the waves. It stays in one place. What’s going on?
When a wave travels across the surface of the sea, the surface of the water simply moves
up and down. The molecules of water are not travelling from one place to another. It is
only the up-and-down movement of the water that is travelling.
We call this up-and-down movement a disturbance. The travelling disturbance forms the
wave that we can see.
Explain why a boat is not pushed along by a wave in the middle
of the sea.
306 SMART SCIENCE
Waves
How can we investigate waves?
In the laboratory we can use a ripple tank to investigate waves.
1. The tank has a glass bottom and contains water.
2. A wooden bar vibrates up and down.
3. The bar makes the surface of the
water move up and down.
It causes a disturbance.
motor
4. The disturbance travels across
the surface of the water.
Tiny waves called ripples
spread out from the bar.
5. The light shining down from the
top of the tank shows up the ripples
as shadows underneath.
light bulb
above tank
vibrating wooden bar
You can measure the speed of the ripples.
Measure the time a single ripple takes to travel from the bar to the end of the tank.
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Measure the distance from the bar to the end of the tank.
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distance
Work out the speed, using speed = time
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taken
How can we show that water waves reflect?
If you place a straight barrier in the path of the ripples, they will bounce off it.
They change direction. They have been reflected.
The red arrows show how the ripples change
direction when they are reflected by the
barrier. The diagram may remind you of the
diagram on page 297, which shows the law of
reflection of light. The barrier is like a mirror.
Sound can also be reflected by a flat surface.
A reflected sound is called an echo.
Light, sound and water ripples all show the same
pattern of reflection. This is one reason why we
say that light and sound travel as waves.
vibrating bar
incident
waves
reflecting
barrier
reflected
waves
The parallel lines represent ripples.
They reflect off the barrier.
Draw a diagram of the tank, bar and barrier showing what you
would see if you looked straight down from above. Add the
ripples. Mark the angle of incidence and the angle of reflection.
(You will have to draw the normal to the reflecting surface.)
PHYSICS 307
Unit 14
How can we represent waves?
Imagine slicing downwards through the
surface of the water in a ripple tank. What
would you see?
In science, we picture ‘ideal’ waves like the
ones shown in the illustration. They go up
and down in a perfectly smooth way. True
waves on the sea or in a ripple tank won’t
have such a perfect shape.
wavelength
crest
distance
trough
This picture may remind you of the diagrams of sound waves on page 286.
We can think of a wave as a series of crests and troughs. The distance from one crest to
the next crest is called the wavelength of the wave.
In a ripple tank, you can change the wavelength of the ripples. To do this, increase the
frequency of the vibrating bar. Then more waves will be produced each second and the
distance from one crest to the next will be less.
1Measure the diagram of the wave. What is its wavelength?
Give your answer in millimetres.
2 How would the ripples in a ripple tank change if you reduced
the frequency of the vibrating bar?
What is special about waves?
The photograph shows
several sets of circular
ripples, spreading out across
the surface of some water.
You can see something
surprising: the ripples pass
right through each other.
Can light waves pass
through each other? Yes.
You can shine two torch
beams so that they cross
– they pass through each
other unaffected.
Similarly, sound waves can pass through each other. All of the instruments in an
orchestra produce their own sound waves, and they all pass through each other to reach
the audience. The audience hears all the instruments together.
308 SMART SCIENCE
Waves
What happens when waves meet?
Look again at the photograph of the circular ripples. If these were waves on the sea and
you were in a boat, you would be pushed up and down by the waves. When two crests
reached you at the same time, you would be pushed up higher than by one wave on its
own. Two troughs together would make you sink lower.
What happens if a crest and a trough arrive at the same time? They cancel out.
Waves in step
Waves out of step
When waves arrive in step with each other, the waves ‘add up’ to make a bigger wave.
If they arrive out of step, they can cancel out.
So waves can add together to make a bigger wave, and they can cancel out to leave
no wave.
Noise-cancelling headphones make use of this. A microphone detects noises from the
outside. The headphones then produce sound waves that are an ‘upside-down’ version
of the noise. The two sets of waves cancel each other out. A person wearing these
headphones can then hear just the music they want to listen to, with no outside noise.
Scientists used to think that light travelled as streams of
particles, like hard, tiny balls. Explain how you could use two
torches to show that light does not behave like this.
PHYSICS 309
Unit 14
What are the two types of wave?
Here is another way to show how waves behave.
1. Stretch out a long spring along the bench.
2. Hold one end fixed and move the other in a regular way.
3. Waves travel along the spring.
There are two ways to make waves on the spring.
Move the end of the spring from side to side. A wave travels along the spring but
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the individual bits of the spring simply move from side to side. This is called a
transverse wave.
Push the end of the spring back and forth, along the length of the spring. Again, a
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wave travels along the spring, but this time the individual bits of the spring move
back and forth along the length of the spring. This is called a longitudinal wave.
Transverse wave
side-to-side
movement
Longitudinal wave
end-to-end movement
A spring can show transverse waves (top) and longitudinal waves (bottom).
Sound travels as a longitudinal wave. Look back at the diagram of how sound travels on
page 288. The loudspeaker moves back and forth horizontally, pushing on the air. The
air molecules push on each other so that the sound travels horizontally too.
Water waves are transverse waves. The surface of the water moves up and down
vertically, while the waves move horizontally.
Light waves are also transverse waves, but it is quite difficult to show this.
If you tie a long rope to a hook on the wall, you can stretch it
out and move the other end to make waves on the rope. Draw
a diagram to show what the waves will look like. Are they
transverse or longitudinal?
310 SMART SCIENCE
Waves
What do waves carry?
Waves carry energy. The energy is transferred from the source to a receiver.
source
waves
receiver
Waves in the sea do not make water travel across the ocean. However, a water wave
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can have enough energy to knock you down when it reaches the beach.
You can hear someone playing a trumpet. Particles of air do not travel all the way from
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the trumpet to your ear. Sound waves bring energy so that you can hear the music.
Light waves transfer energy from a source such as a sealed light bulb. The particles
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of air do not move from the bulb to you. Your eyes detect the light because of the
energy transferred from the bulb to your eyes.
Waves can also carry information.
Speech is carried by sound waves. You use your vocal folds to change the amplitude
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and frequency of the sound waves you make. A listener’s ears receive the sound
waves. The listener’s brain recognises the changes in amplitude and frequency, so
that they can understand what is being said.
When you see things, light enters your eyes. Your eyes send electrical signals to your
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brain. Your brain interprets the signals, so that you know what you are looking at.
Find out about other ways in which humans use waves to
transfer information.
Earthquakes are caused by sudden releases
of energy inside the rocky surface (crust)
of the Earth. For example, two pieces of
the crust may slip over each other.
Buildings near the site of an earthquake
may move up and down or from side to
side for up to a minute. If the earthquake
is strong enough, the energy released
can move the buildings so much that
they collapse.
Suggest how energy released by earthquakes is transferred to
buildings. How can we protect buildings from damage?
PHYSICS 311
Unit 14
You need to remember that:
A ripple tank can be used to show the behaviour of waves on water.
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The wavelength of a wave is the distance from one crest to the next.
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Waves are of two types, transverse (such as water waves and light) and longitudinal
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(such as sound waves).
Waves can pass through each other.
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When waves meet, their effects can add together or cancel out.
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Waves can transfer energy and information.
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Next time
Unit 16 Energy can be transferred from hot objects as infra-red waves (page 317).
Unit 21 Astronomers observe the Universe by using telescopes to detect many types of waves (page 343).
312 SMART SCIENCE