# 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 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 or to the bottom of a . 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. is also measured in metres ($$m$$) - it is a length after all.

The ($$f$$) 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 ($$Hz$$), 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