Fabry-Perot Interferometer

angle θ

A Fabry–Perot interferometer (also called an étalon) is an optical instrument consisting of two highly reflecting parallel mirrors separated by a transparent plate of width

d

. This produces a multiple-beam interference pattern from a beam of light of wavelength

λ

with an angle

θ

. The light interferes constructively whenever

mλ=2dcosθ

, where

m=1,2,3,…

.