Foucault Method

a method of measuring the speed of light. The Foucault method consists in successive reflections of a light beam, first from a rapidly rotating mirror, then from a fixed mirror located at a precisely measured distance, and then again from the first mirror, which turned through a certain small angle while the beam traveled from and to it. If the speed of rotation of the first mirror and the distance between the two mirrors are known, the speed of light is determined from the shift in the direction of the triply reflected light beam. The method was first used to measure the speed of light in air by J. B. L. Foucault in 1862. (See alsoSPEED OF LIGHT.)