Wave number

Where is the wavelength, is the frequency, is the speed (of the wave) and 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 .