Two types of wave

Wave motion can be split into two different types. The easiest to picture is the motion in which the wave travels forward and the material which forms the wave moves from side to side. Waves on water are a good example of this 'transverse' wave type.

See a video showing that water waves vibrate at right angles to the direction of movement (814k).

Sometimes the direction of the to and fro movement which forms a wave is the same as the general direction of the wave itself. Sound is an example of this 'longitudinal' wave type. The vibration which sets up the sound wave moves the air molecules to and fro. Each time this happens, the air molecules move neighbouring air molecules which in turn move further air molecules. The material forming the wave moves in the same direction as the wave itself. Something similar can be seen in a queue of traffic at traffic lights. As a group of cars is permitted to move on, a wave front moves back down the queue as each car moves forward.

Measuring waves

Four measurements are commonly made for waves. They are the height of the wave, the distance between one wave and the next, how often waves pass by a single point and the speed at which the wave is moving. These are given the names amplitude, wavelength, frequency and velocity.

The diagram gives some indication of these measurements and more details are given later in their individual sections.

Amplitude of a wave

The amplitude measures how 'far' the wave moves from its rest position. In strings or springs, this would be measured in centimetres or metres. For electrical circuits, waves might be seen as a varying voltage - here the amplitude would be measured in volts.

The size of a wave on water is measured from the wave peak to the position of the water when there are no waves. The diagram shows this measurement being made for water.