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Wavelength is a property of a phenomenon that regularly repeats in space (as opposed to time). The wavelength of such a phenomenon is the distance over which the phenomenon repeats. In the SI system of units, wavelength is measured in metres (m).

Wavelength ( λ ) can be determined from the wave speed ( c ) and frequency ( f ):

For electromagnetic waves propagating in a vacuum, c = 299,792,458 m/s = 300 Mm/s (the speed of light). Rounding to convenient values for radio waves, 300 divided by the frequency in megahertz (Mhz) gives the wavelength in meters:

Thus FM radio waves, with a frequency of roughly 100 MHz, have a wavelength of around 3 m, while the wavelength of AM radio waves (roughly 1000 kHz or 1 MHz frequency) have much longer wavelengths, around 300 m.

Three systems of designating ranges of electromagnetic wave frequencies and wavelengths are in common use:

For visible light, wavelengths are several hundred nanometres, or several tenths of a micrometre.