For a Fabry-Perot or laser cavity,
wavenumber interval between successive longitudinal cavity
modes of vibration;
\(\Delta \tilde{\nu} = 1/(2nl)\), where
\(l\) is the cavity spacing and
\(n\) is the refractive index of the material in the cavity.
Note:
SI unit: \(\pu{m-1}\).
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 747 (https://doi.org/10.1515/pac-2019-0203)