Properties of Waves » Transverse and Longitudinal Waves

What is a wave?

Drop a pebble into a pond and ripples spread out across the surface. No water travels from the splash to the bank, yet something clearly gets there. That something is energy.

Every wave starts with a vibration: something moving backwards and forwards about a fixed point, again and again. The vibration is passed on from one place to the next, and this travelling pattern of vibrations is the wave.

Key terms:

• Vibration: a repeated movement backwards and forwards about a fixed position. It is also called an oscillation.
• Wave: a series of vibrations that transfers energy and information from one place to another without transferring matter.
• Transverse wave: a wave in which the vibrations are perpendicular (at 90°) to the direction in which the wave travels.
• Longitudinal wave: a wave in which the vibrations are parallel to the direction in which the wave travels.
• Compression: a region of a longitudinal wave where the particles (or coils) are squashed closer together than normal.
• Rarefaction: a region of a longitudinal wave where the particles (or coils) are spread further apart than normal.

Waves transfer energy and information, not matter

This is the big idea of the whole topic, and the exam board expects you to know it word for word: waves transfer energy and information without transferring matter.

Picture a ball floating on a pond. When ripples pass, the ball bobs up and down. It is not carried along to the bank. After the ripples have gone, the ball is in the same place as before. The water under the ball has only moved up and down as well. The wave travelled across the pond, but the water (the matter) did not.

The ball also shows that the wave carries energy. Before the ripples arrived the ball was still. The wave made it move, so energy must have been transferred to it from the place where the pebble landed.

A crowd wave in a sports ground is a good picture of this. Each person stands up and sits down again in turn. The wave races all the way around the ground, but every person finishes in their own seat.

Transverse waves

In a transverse wave, the vibrations are perpendicular to the direction the wave travels. Perpendicular means at right angles, or at 90°.

You can make one with a long rope. Tie one end to a post, hold the other end and flick your hand up and down. A wave travels along the rope towards the post, but each piece of rope only moves up and down. Up and down is at 90° to along, so the wave is transverse.

A slinky spring shows the same thing. Stretch the slinky along a table with a friend holding the far end still. Shake your end from side to side across the table. A wave travels along the slinky, while each coil moves from side to side.

Examples of transverse waves: light waves, ripples on water and waves on a rope.

To draw a transverse wave: draw a smooth wavy line that rises and falls evenly, like a row of hills and valleys. Add a long arrow pointing along the wave and label it 'direction of travel'. Add a short double-headed arrow pointing up and down and label it 'vibrations'. The two arrows must be at right angles.

Longitudinal waves

In a longitudinal wave, the vibrations are parallel to the direction the wave travels. The vibrations are backwards and forwards along the same line that the wave moves along.

Use the slinky again. This time push and pull your end, towards your friend and back. A bunched-up group of coils travels along the slinky. This squashed region is a compression. Behind it comes a stretched region where the coils are further apart than normal. This is a rarefaction. As you keep pushing and pulling, compressions and rarefactions follow each other along the spring.

Tie a piece of ribbon to one coil and watch it. The ribbon moves backwards and forwards about its starting place, in line with the slinky. It does not travel to the far end. Once again, the wave moves along but the matter does not.

Example of a longitudinal wave: sound waves. When sound travels through air, the air particles vibrate backwards and forwards along the direction the sound is travelling, making compressions and rarefactions in the air. You will study sound in the lesson Sound Waves.

To draw a longitudinal wave: draw a row of short vertical lines. Put some groups of lines close together and label one group 'compression'. Put the lines between them far apart and label one of these regions 'rarefaction'. Add a long arrow for the direction of travel and a short double-headed arrow pointing the same way for the vibrations.

Comparing the two types of wave

	Transverse wave	Longitudinal wave
Direction of the vibrations	Perpendicular (at 90°) to the direction the wave travels	Parallel to the direction the wave travels
What it looks like	A wavy line that rises and falls	Compressions and rarefactions
Slinky demonstration	Shake the end from side to side	Push and pull the end
Examples	Light, water ripples, waves on a rope	Sound
Transfers energy and information?	Yes	Yes
Transfers matter?	No	No

Units for the Waves topic

You have used the metre, the second and the metre/second since Speed, Distance and Time. This topic adds two more units, the hertz and the degree. Each quantity in the table is defined in the lessons that follow. For now, learn which unit goes with which measurement.

Unit	Symbol	What it will measure in this topic
hertz	Hz	Frequency
metre	m	Wavelength, amplitude and distances
metre/second	m/s	Wave speed
second	s	Time and time period
degree	°	Angles, such as the angles light makes when it is reflected or refracted

Exam-style question

Two students stretch a slinky spring along a bench. A small piece of ribbon is tied to one coil near the middle.

(a) Describe how one student should move her end of the spring to send a longitudinal wave along it. [1 mark]

(b) Describe the movement of the ribbon as the longitudinal wave passes. [2 marks]

(c) Explain the difference between a transverse wave and a longitudinal wave. [2 marks]

(d) A cork floats on a lake. Water waves pass the cork. State what happens to the cork and explain what this shows about waves. [2 marks]