Longitudinal cavity mode
= Longitudinal cavity mode
{title2=$\Delta\nu=c/(2L)$}
A <normal mode> whose longitudinal phase satisfies the resonator's round-trip condition. Holding the transverse mode fixed, an empty nondispersive two-mirror cavity of length L has adjacent optical <frequencies> separated by $c/(2L)$. An offset from mirror or transverse phases changes the position of the mode sequence without changing this ideal spacing. A dispersive cavity instead uses its round-trip group delay.