Spectral extinction between crossed polarizers
= Spectral extinction between crossed polarizers
{title2=$\Gamma=m\lambda$}
A birefringent plate with retardation $\Gamma=t\Delta n$ gives $I/I_P=\sin^2(2\alpha)\sin^2(\pi\Gamma/\lambda)$. At non-extinction azimuths, wavelengths satisfying $\Gamma=m\lambda$ are removed by a crossed analyser. At a principal-axis azimuth every wavelength is extinguished.