Interference colour

Colour produced when wavelength-dependent interference selectively transmits or reflects part of white light. Between crossed polarizers, a uniform birefringent plate gives a factor ${\sin }^2(\pi \Gamma /\lambda )$, where $\Gamma$ is its optical path difference. The observed colour also depends on illumination and the spectral response of the observer.

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.