Waves

Waves carry energy from one place to another. Think about the energy released when waves break onto a beach or the immense energy carried along by a tidal wave or tsunami. These waves make the material in which they are travelling vibrate as they pass through it. Experiments with the slinky spring and the water tank will have shown you that vibrations make waves. The individual particles of the material do not move along, they simply vibrate as the wave energy is passed to them.

For example a fishing float or a cork will simply bob up and down as a water wave passes it – the float does not move along in the direction that the wave is moving. Waves in the sea are quite complicated especially in shallow water. Because of the friction between the water and the sea bed the water particles move in circles and that is why waves 'break' as the depth of the water gets less and less.

Wave definitions

There are a few important definitions that describe waves and the way they move:

WAVELENGTH (λ)

- the distance from one crest to another or one trough to another. (In fact generally from any point on the wave to the next exactly similar point)

FREQUENCY (f)

- the number of vibrations of any part of the wave per second. The bigger the frequency the higher the pitch of the note or the bluer the light

TIME PERIOD (T= 1/f)

- the time for one oscillation to take place. The larger the time period the lower the frequency of the wave.

AMPLITUDE (A)

- the maximum distance that any point on the wave moves from its mid position. The bigger the amplitude the louder the sound, the rougher the sea or the brighter the light

SPEED (v)

- how fast the wave transfers energy from one place to another. This is about 330 m/s for sound in air and a massive 300 000 000 m/s for light in space.

Longitudinal and transverse waves

There are two distinct types of wave and they are shown in the two diagrams.

(a) TRANSVERSE WAVES

In a transverse wave the vibration is side to side, ACROSS the direction in which the wave is moving. Examples of this type of wave are water waves, shaking a slinky, radio, light, X rays, gamma rays, microwaves, infrared, ultra violet and radar, S waves in earthquakes.

(b) LONGITUDINAL WAVES

In a longitudinal wave the vibration is backwards and forwards, ALONG the direction in which the wave is moving. Examples of this are sound waves in air, P waves in earthquakes and a quick push along the slinky.

The speed, frequency and wavelength of a wave are connected by the following formula: