In
physics,
wavelength is the distance between repeating units of a propagating
wave of a given
frequency. It is commonly designated by the
Greek letter ''
lambda'' (λ). Examples of wave-like phenonomena are
light,
water waves, and
sound waves.
In a wave, a property varies with the position. For example, this property can be the air
pressure for a
sound wave, or the magnitude of the
electric or the
magnetic field for
light. The wavelengths of frequencies audible to the human ear (20 Hz–20 kHz) are between approximately 17 m and 17 mm, respectively. Visible light ranges from deep red, roughly 700 nm to violet, roughly 400 nm (430–750 THz). For other examples, see
electromagnetic spectrum.
Relationship with frequency
Wavelength ''λ'' is inverse proportional with the
frequency ''ν'' (Greek "nu"), the number of wave periods per time unit passing a given point, as in
:
where
is the propagation velocity of the wave. In the case of
electromagnetic radiation, such as light, in a vacuum, this speed is the
speed of light, 299,792,458 m/s or about
109 km/h. For
sound waves in air, this is the
speed of sound, 344 m/s (1238 km/h) in air at room temperature. Usually,
SI units are used, where the wavelength is expressed in
meters, the frequency in
Hz, and the propagation velocity in meters per second.
In non-vacuum media
The speed of light in most media is lower than in vacuum, which means that the same frequency will correspond to a shorter wavelength in the medium than in vacuum. The wavelength in the medium is
:
where ''n'' is the
refractive index of the medium. Wavelengths of electromagnetic radiation are usually quoted in terms of the vacuum wavelength, unless specifically indicated as the "wavelength in the medium". In acoustics, unless otherwise specified, the term wavelength is used to mean the wavelength in the medium.