Frequency, wavelength, amplitude and wave speed

You need to know about these quantities used in waves - what they mean, the symbols used for them and the units used to measure them.

Sine wave with crests and troughs labelled. Their distance from the centre line (amplitude) is labelled 'a'. The distance between two crests or two troughs (wavelength) is labelled with the lambda symbol.Sine wave showing peaks, troughs and wavelength

The amplitude (\(a\)) of a wave is the distance from the centre line (or the still position) to the top of a crest or to the bottom of a trough. Be careful with this quantity - the centre line is not always given in a diagram. Amplitude is measured in metres (\(m\)). The greater the amplitude of a wave then the more energy it is carrying.

The wavelength, \(\lambda\), of a wave is the distance from any point on one wave to the same point on the next wave along. (The symbol is a Greek letter, 'lambda'.) To avoid confusion, it is best to measure wavelength from the top of a crest to the top of the next crest, or from the bottom of a trough to the bottom of the next trough. Wavelength is also measured in metres (\(m\)) - it is a length after all.

The frequency(\(f\)) of a wave is the number of waves passing a point in a certain time. We normally use a time of one second, so this gives frequency the unit hertz (\(Hz\)), since one hertz is equal to one wave per second.

Don't get confused with this quantity frequency. It is not a distance travelled by waves, nor is it a speed, although it is linked to both of these quantities. For water waves and sound waves the unit hertz is usually good enough but radio and TV waves have such a high frequency that the kilohertz (\(kHz\)) or even the megahertz (\(MHz\)) are better units.

\[1kHz = 1,000Hz\]

\[1MHz = 1,000,000Hz\]

The speed (or sometimes you might see it called velocity) of a wave (\(v\)) is how far the wave travels in a certain time.

Wave speed is measured in metres per second (\(m\,s^{-1}\)).

All the electromagnetic waves travel at 300,000,000 metres per second (\(3\times 10^{8}\,m\,s^{-1}\)). Sound travels at about 340 metres per second.

Sound waves