Wave number

$k=\backslash \backslash frac\{2\backslash \backslash pi\}\{\backslash \backslash lambda\}=\backslash \backslash frac\{2\backslash \backslash pi\backslash \; u\}\{c\}=\backslash \backslash frac\{\backslash \backslash omega\}\{c\}.$

Where $\backslash \backslash lambda$ is the wavelength, $\backslash \; u$ is the frequency, $c$ is the speed (of light) and $\backslash \backslash omega$ is the angular frequency.

Sometimes the wavenumber is defined as .

In the case of molecules, when an electron jumps from a higher orbit to a lower orbit energy is given out in the form of spectral lines. The wave number of the emitted line for a Hydrogen atom is given by

where λ is the wavelength, R is Rydberg constant,

and  are the orbit numbers, and  is greater than .