wavelength

Watch ocean swells rolling in: the distance from the top of one swell to the top of the next is the wavelength. It is simply how long one full ripple of a wave is. The question it answers is: how much space does a single complete cycle of the wave take up?

The wavelength, written with the Greek letter lambda, is the distance between two consecutive points that are in the same part of the cycle, crest to crest, trough to trough, or compression to compression. It is measured in metres. It links to the wave speed v and the frequency f by the fundamental wave relation v = f lambda, so lambda = v / f. For a fixed speed, a higher frequency means a shorter wavelength.

Wavelength sets the character of a wave: red light has a longer wavelength than blue light, and a low bass note has a longer wavelength than a high whistle. An honest note: the wavelength depends on the medium, because the speed does. When light passes from air into glass it slows down and its wavelength shrinks, even though its frequency (and so its colour) stays the same.