What are Waves?
Introduction

You are most likely to have come across waves in water, especially the sea. There are many other waves present in nature. For example, sound is transmitted from one place to another by sound waves. Light also travels as a wave.
Waves and energy
Ocean waves can be very destructive. They carry a lot of
energy
Energy is the capacity to do work. The SI unit of energy is the joule (J).
energy
.

Figure 1.   Powerful waves on the sea.
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When a wave strikes the shore, the energy in the waves is unleashed and can cause erosion of the coastline or destruction of property. In Fig.1 above, the rocks absorb the energy. The waves also give energy to a boat by making it move up and down. During a storm, the energy in the waves may be enough to damage the boat.

What type of energy do ocean waves have?
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Imagine if we could turn this destructive energy of waves into a more useful form, such as electricity. Engineers are trying hard to find ways of harnessing the energy of the seas.

Even though sea waves have travelled hundreds of miles over the ocean, the water molecules themselves do not actually move very far. If you watch a log floating on the sea, it bobs up and down as the waves go past, but does not rush in with the wave.

A stadium wave (or Mexican wave) is a great example of a wave. The energy (of the people standing up) is transferred around the stadium, but the people themselves do not move very far at all.

Figure 2.   A Mexican wave.
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When a drummer hits a cymbal, it shakes and vibrates. It has
kinetic energy
Kinetic energy is the energy associated with motion. A fast moving object has more kinetic energy than an otherwise identical slow moving object.
kinetic energy
. The energy is transferred to the air molecules around the cymbal. The sound that reaches your ears is a vibrating wave of air molecules. Fig.3 below shows a sound wave moving from a cymbal to a person's ear.

Figure 3.   Sound waves from a vibrating cymbal.
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Fill in the blanks in this sentence to describe how the energy is transferred

  • The energy of the vibrating passes to the air molecules. The energy in the air spreads out and is detected by your as .
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The air molecules do not move from the cymbal to your ear. If they did, you would feel a breeze every time you heard a sound. The molecules just pass on the vibrations to their neighbours. When the sound reaches you, the vibrations are passed on to your eardrum. Your brain knows that a vibrating eardrum means there is a sound.

Light also behaves like a wave. When light waves from the sun fall on your body, you feel them as heat. The energy is transferred millions of miles from the sun, through empty space to the Earth.

As well as passing energy, waves can be used to send information. Imagine you hold one end of a string and a friend holds the other end. You could communicate simple messages back and forth using the string. One flick of the string could mean 'Yes' and two, 'No'. Televisions, radios, mobile phones, and satellites also use waves to transmit information.
Types of waves
Look at the wave in the string of beads below in Fig.4. This is made by shaking the beads up and down.

Figure 4.   A wave made by shaking beads on a string.
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Which way do the individual beads on the string move?
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In which direction is the energy transferred?
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Do the beads move in the same direction as the energy transfer?
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The wave in the string above is a transverse wave. This type of wave occurs whenever the material of the wave moves at right angles to the direction in which the wave itself is moving.

The wave in Fig.5 is made by compressing the spring.

Figure 5.   A wave made by squeezing a spring.
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Which way do the individual coils of the spring move?
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Do the coils of the spring move in the same direction as the wave?
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The movement of the material (the coils) is in the same direction as the motion of the wave. This type of wave is called a longitudinal wave. It is a wave of squeezing and stretching, or compression and rarefaction.

Could you have a
transverse wave
In a transverse wave, the material moves at right angles to the direction of motion of the wave. Ripples on a pond are an example of transverse waves.
transverse wave
on a spring?
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Could you have a
longitudinal wave
In a longitudinal wave, the material moves in the same direction as the motion of the wave. Sound is an example of a longitudinal wave.
longitudinal wave
on a rope?
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What type of wave is the stadium (or Mexican) wave in Fig.2?
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Figure 6.   A wave forming in traffic.
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What type of wave is shown in the traffic in Fig.6?
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Click on the figure below to interact with the model.

 Figure 7.  Bouncing balls forming a wave.

What type of wave is made by the bouncing balls in Fig.7?
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Studying waves
Surface waves on water are a good way to study the behaviour of transverse waves.
Figure 8.   A ripple tank is a good way to study waves.
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The waves produced in the ripple tank make shadows underneath. It is easier to make measurements on the shadows than on the waves themselves.

Real water waves
It is simple to think of surface waves on water as being transverse waves. However, the motion of the water molecules themselves is more complicated than a simple up and down motion. Look at the water wave in Fig.9. The balls show how the water molecules are moving.

Figure 9.   Real waves on water.
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How do the water molecules in a real water wave move?
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Summary

Waves are a means of transferring energy, without actually transporting matter.

Waves can be either transverse or longitudinal.

Transverse waves can be set up on a string or on the surface of water. For this type of wave, the disturbance is at a right angle to the movement of the wave.

Longitudinal waves can be set up on a spring, in the air when a sound wave passes, or in heavy traffic. For this type of wave, the disturbance is in the same direction as the movement of the wave.
Exercises
Figure 10.  
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1. A transverse wave moves through the beads shown in Fig.10 above. How do the beads move as the wave passes?
Figure 11.  
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2. A longitudinal wave moves through the beads shown in Fig.11 above. How do the beads move as the wave passes?
3. Decide whether each of these waves are longitudinal, transverse or neither.
  • Mexican wave:
    Sound wave:
    Wave in traffic:
    Real water wave:
    Heat wave:
    Wave on a spring when it is shaken from side to side:
    Wave on a spring when it is compressed and stretched:
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4. What type of wave is represented in Fig.12 below?
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Figure 12.  
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5. What is happening at points 1 and 2 in Fig.12?
  • Point 1
    Point 2
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