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)