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Fabry–Perot interferometer(fah-bree pair-oh ) A type of interferometer used in spectroscopy to study the fine structure within a spectrum. It has extremely high resolution. It consists essentially of two flat plates, separated and held parallel to each other very accurately. A beam of light, infrared, etc., is multiply reflected in the gap between the inner surfaces, so the plate material must have high reflectivity and low absorption in the wavelength region of interest. The gap is normally precisely controlled and adjustable.
The Fabry–Perot can be thought of as acting like a tunable filter. The gap (d) is related to the wavelength (λ) passed with maximum efficiency by the following equation: 2d cosθ = m λ, where m is an integer known as the order number and θ is the angle of incidence of the beam on the plates (usually arranged to be zero so that cosθ = 1). When the beam is multiply reflected, the particular wavelengths that satisfy the above equation are partially transmitted. This transmitted radiation is focused by a lens or mirror system onto a detector or screen to produce an interference pattern consisting of narrow sharp bright rings on a dark background. The instrument is able to distinguish, or resolve, two very close wavelengths as two almost overlapping superimposed ring patterns, giving it its high resolving power. The wavelength producing the second ring pattern satisfies the equation for the same value of the gap (d) as the first wavelength, but with a different order number (m ). The ring patterns can be separated by the use of a diffraction grating or dispersion by the prism of a spectrograph, which acts as an order sorter. Instruments using two or three Fabry–Perots in series have even higher spectral resolutions.