Editing Fabry–Pérot interferometer (section)

== Basic description ==
[[File:Fabry Perot Interferometer - diagram.png|thumb|350px|Fabry–Pérot interferometer, using a pair of partially reflective, slightly wedged optical flats. The wedge angle is highly exaggerated in this illustration; only a fraction of a degree is actually necessary to avoid ghost fringes. Low-finesse versus high-finesse images correspond to mirror reflectivities of 4% (bare glass) and 95%.]]
The heart of the Fabry–Pérot interferometer is a pair of partially reflective glass [[optical flat]]s spaced micrometers to centimeters apart, with the reflective surfaces facing each other. (Alternatively, a Fabry–Pérot ''etalon'' uses a single plate with two parallel reflecting surfaces.) The flats in an interferometer are often made in a wedge shape to prevent the rear surfaces from producing interference fringes; the rear surfaces often also have an [[anti-reflective coating]].

In a typical system, illumination is provided by a diffuse source set at the [[focal plane]] of a [[collimating lens]]. A focusing lens after the pair of flats would produce an inverted image of the source if the flats were not present; all light emitted from a point on the source is focused to a single point in the system's image plane. In the accompanying illustration, only one ray emitted from point A on the source is traced. As the ray passes through the paired flats, it is repeatedly reflected to produce multiple transmitted rays which are collected by the focusing lens and brought to point A' on the screen. The complete interference pattern takes the appearance of a set of concentric rings. The sharpness of the rings depends on the reflectivity of the flats. If the reflectivity is high, resulting in a high [[Q factor]], [[monochromatic light]] produces a set of narrow bright rings against a dark background. A Fabry–Pérot interferometer with high Q is said to have high ''finesse''.