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In science, the wavenumber indicates the number of wavelengths that would fit in a unit of length. The normal units for wavenumbers are inverse centimeters cm-1. A different name for this unit is kayser (after Heinrich Kayser). Light with a wavelength of 500 nm (green) has a wavenumber of 20,000 cm-1 or 20 kK. Photon energy and frequency are proportional to wavenumber: 10 kK corresponds to 1.24 eV.

Historically, wavenumbers were introduced by Janne Rydberg in the 1880's in his analyses of atomic spectra.

Wavenumbers (), wavelength (), and frequency () are related: