In optics, a Gires–Tournois etalon is a transparent plate with two reflecting surfaces, one of which has very high reflectivity, ideally unity. Due to multiple-beam interference, light incident on a Gires–Tournois etalon is (almost) completely reflected, but has an effective phase shift that depends strongly on the wavelength of the light.

The complex amplitude reflectivity of a Gires–Tournois etalon is given by

${\displaystyle r=-{\frac {r_{1}-e^{-i\delta }}{1-r_{1}e^{-i\delta }}}}$

where r₁ is the complex amplitude reflectivity of the first surface,

${\displaystyle \delta ={\frac {4\pi }{\lambda }}nt\cos \theta _{t}}$

n is the index of refraction of the plate

t is the thickness of the plate

θ_t is the angle of refraction the light makes within the plate, and

λ is the wavelength of the light in vacuum.