The 2 Types of Wave: Longitudinal and Transverse

“In Space, no-one can hear you scream” was the tagline for the classic science fiction horror movie Alien. It is both right and wrong at the same time as you will find out here.

You come across waves every day. Although there are many different ones, they can all be classified as transverse waves or longitudinal waves. They are vibrations that transfer energy but not matter from one place to another.

Landslip caused by seismic waves.

Landslide caused by Seismic waves [photo: USGS]

1. Longitudinal Waves

The first thing that you should know about longitudinal waves is that they need a medium (a substance – solid, liquid or gas) to travel through. As the wave passes, the particles of the medium oscillate backwards and forwards, vibrating in the same direction as the wave is travelling. The particles do not move anywhere overall. Once the wave passes, they are in the same place as they were before.

To help you to think of this type of wave, imagine holding one end of a long stretched-out spring with one of your friends holding the other end. If you then imagine moving your hand backwards and forwards – NOT SIDEWAYS – the spring will be alternately stretched and compressed. These stretches and compressions will travel along the spring and your friend will feel them.

Energy Transfer

The energy from your movements has therefore been transferred from one place to another by a wave travelling through the spring. Once you have stopped, all of the coils of the spring and the spring itself will still be in the same position as before.

Compressions and Rarefactions

Particles of solids, liquids and even gases act like the coils of a spring. In some parts of the wave, the particles will be closer together, we call that a compression. In other parts of the wave, the particles will be spread apart, we call those areas rarefactions. The wavelength of this type of wave is measured from the centre of a compression (or rarefaction) to the centre of the next compression (or rarefaction). The frequency measures how many complete wavelengths pass in every second.

Examples of longitudinal waves are sound and seismic waves (P-waves) – how do you think that you could you use your senses to detect them? What other things could you use to detect these waves?

2. Transverse Waves

Transverse waves can travel through substances but they do not have to. Water waves and S-waves from earthquakes are two examples of transverse waves that only travel in a medium. Other transverse waves can travel through a vacuum as well as through substances. These are the electromagnetic waves – light, radio waves, X-rays and others.

Transverse wave.

Transverse wave [image: LucasVB]

Side to Side

To help you to think of this type of wave, imagine once again the long stretched-out spring with one of your friends holding the other end. If you then imagine moving your hand from side to side, the spring will also move from side to side forming ‘s’ shaped curves with a series of peaks and troughs. So in a transverse wave, the vibrations are at 90o to the direction in which the wave is travelling. After the waves have passed, the spring is still there, in its original position. The wavelength is measured from one peak (or trough) to the next. The frequency is the number of complete waves passing in each second.

With waves on the sea and earthquake waves, it is easy to know what is vibrating but what is it that is vibrating with light, X-rays and other related waves? The clue is in the name, they are all members of the electromagnetic spectrum. Throughout space, there are electrical and magnetic fields and it is those fields that vibrate as light waves. radio waves, microwaves and the others pass through.

Measuring Waves

The units of wavelength vary, it could be metres, centimetres or any other unit of length. Frequency is always measured in Hz (Hertz). One Hz is one complete cycle of the wave passing in one second. That will be one compression-rarefaction-compression passing in one second for longitudinal waves or peak-trough-peak for transverse waves.

And finally …

We mentioned at the start that “in space no-one can hear you scream” is both right and wrong at the same time. Should you find yourself in space without a spacesuit and were able to survive long enough to let out a scream, indeed no-one would be able to hear you as sound is a longitudinal wave and cannot travel through the vacuum of space. However, if you were still in a spacesuit and your radio was on, radio waves are electromagnetic waves which can travel through space, so your scream would be heard loud and clear!

Taught science for 16 years at a secondary school in the East Midlands.