Free spectral range
= Free spectral range
{title2=$\Delta\lambda_{\rm FSR}$}
{wiki}
The <free spectral range> is the <wavelength> or <frequency> spacing between adjacent orders or resonances of an <optical instrument>. It identifies when different orders can overlap. Always specify the coordinate: a uniform spacing in <frequency> need not be uniform in <wavelength>. For an ideal <Fabry–Pérot interferometer> at <normal incidence>, with mirror separation $d$ and constant <refractive index> $n$, <Fabry–Pérot resonance spacing> gives $\Delta\nu_{\rm FSR}=c/(2nd)$, where $c$ is the <speed of light> in <vacuum>. The <free spectral range of an echelle grating> follows a different order condition.