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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 λ has an inverse relationship to frequency
f, 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, dealing with signals (waves) in air we have the speed of sound in air, so the conversion becomes,
-
where:
- λ = wavelength of an electromagnetic wave or
- λ = wavelength of a sound wave
- c = speed of light in vacuum = 299,792.458 km/s ~ 300,000 km/s = 300,000,000 m/s or
- c = speed of sound in air = 343 m/s at 20 °C (68 °F)
- f = frequency of the wave
For radio waves this relationship is easily handled with this formula: meters of wavelength = 300/frequency in megahertz
(MHz)
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:
-
where λ0 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
-
where 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
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