 The heart of the Fabry – Pérot interferometer is a pair of partially silvered glass optical flats spaced several millimeters to centimeters apart with the silvered surfaces facing each other . ( Alternatively , a Fabry – Pérot etalon uses a transparent plate with two parallel reflecting surfaces . ) As with the Fizeau interferometer , the flats are slightly beveled . In a typical system , illumination is provided by a diffuse source set at the focal plane of a collimating lens . A focusing lens produces what would be an inverted image of the source if the paired flats were not present ; i.e. in the absence of the paired flats , all light emitted from point A passing through the optical system would be focused at point A ' . In Fig . 6 , only one ray emitted from point A on the source is traced . As the ray passes through the paired flats , it is multiply 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 ( i.e. high finesse ) , monochromatic light produces a set of narrow bright rings against a dark background . In Fig . 6 , the low @-@ finesse image corresponds to a reflectivity of 0 @.@ 04 ( i.e. unsilvered surfaces ) versus a reflectivity of 0 @.@ 95 for the high @-@ finesse image . 
