Definition
It can be defined as either
- , the number of wavelengths per unit distance, where λ is the wavelength, sometimes termed the spectroscopic wavenumber, or
- ,the number of wavelengths per 2π units of distance, sometimes termed the angular or circular wavenumber, but more often simply wavenumber.
Its usual symbols are, σ or k, the first three used for one definition, the last for another. It has dimensions of reciprocal length, so its SI unit is m−1 and cgs unit cm−1 (in this context formerly called the kayser, after Heinrich Kayser).
For electromagnetic radiation in vacuum, wavenumber is proportional to frequency and to photon energy. Because of this, wavenumbers are used as a unit of energy in spectroscopy. In the SI units, wavenumber is expressed in units of reciprocal meters (m−1), but in spectroscopy it is usual to give wavenumbers in reciprocal centimeters (cm−1). The angular wavenumber is expressed in radians per meter (rad·m−1).
Read more about this topic: Wavenumber
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