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Fabry-Perot Interferometer

A Fabry-Perot interferometer is very similar to a Fabry-Perot filter. However, instead of a thin layer of some medium sandwiched between two mirrors, the Fabry-Perot interferometer has two parallel, highly reflective mirrors separated by an air gap several millimeters to several centimeters in width.

Light of wavelength λ constructively interferes when the following relationship is satisfied:

mλ = 2d cosθ

where m is the order and θ is the angle of the incoming light.

If one of the mirrors is mechanically moved back and forth (most systems use a piezoelectric crystal), the wavelength that constructively interferes at a particular angle θ varies, and the system acts as an interferometer. Since the angle of refraction also depends on the index of refraction of the gap between the mirrors, Fabry-Perot interferometers can also be used with constant d but by changing the pressure of some filler gas.