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The **wavelength** is the distance between repeating units of a wave pattern. It is commonly designated by the greek letter *lambda* (λ).

In a sine wave, the wavelength is the distance between peaks:

The *x* axis represents distance, and *I* would be some varying quantity (for instance air pressure for a sound wave or strength of the electric or magnetic field for light), at a given point in time as a function of *x*.

Wavelength has an inverse relationship frequency, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave divided by the frequency of the wave. When dealing with electromagnetic radiation in a vacuum, this speed is the speed of light *c*, so the conversion becomes,

- λ = wavelength of an electromagnetic wave
*c*= speed of light = 3×10^{8}m/s*ν*= frequency of the wave

When light waves (and other electromagnetic waves) enter a medium, their wavelength is reduced by a factor equal to the refractive index *n* of the medium, but the frequency of the wave is unchanged. The wavelength of the wave in the medium, λ' is given by:

_{0}is the vacuum wavelength of the wave. Wavelengths of electromagnetic radiation are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated.

Louis-Victor de Broglie discovered that all particles with momentum have a wavelength, called the **de Broglie wavelength**. For a relativistic particle, this wavelength is given by

*h*is the Planck constant,

*p*is the particle's momentum,

*m*is the particle's mass, and

*v*is the particle's velocity.

See also: frequency, period, amplitude

**Wavelength** is the title of a 1978 album by Van Morrison.