Fizeau-Foucault apparatus

Fizeau–Foucault apparatus is a term sometimes used to refer to two types of instrument historically used to measure the speed of light. The conflation of the two instrument types arises in part because Hippolyte Fizeau and Léon Foucault had originally been friends and collaborators. They worked together on such projects as using the Daguerreotype process to take images of the Sun between 1843 and 1845 and characterizing absorption bands in the infrared spectrum of sunlight in 1847.

In 1834, Charles Wheatstone developed a method of using a rapidly rotating mirror to study transient phenomena, and applied this method to measure the velocity of electricity in a wire and the duration of an electric spark. He communicated to François Arago the idea that his method could be adapted to a study of the speed of light. Arago expanded upon Wheatstone's concept in an 1838 publication, emphasizing the possibility that a test of the relative speed of light in air versus water could be used to distinguish between the particle and wave theories of light.

In 1845, Arago suggested to Fizeau and Foucault that they attempt to measure the speed of light. Sometime in 1849, however, it appears that the two had a falling out, and they parted ways pursuing separate means of performing this experiment. In 1848−49, Fizeau used, not a rotating mirror, but a toothed wheel apparatus to perform an absolute measurement of the speed of light in air. In 1850, Fizeau and Foucault both used rotating mirror devices to perform relative measures of the speed of light in air versus water. Foucault used a scaled-up version of the rotating mirror apparatus to perform an absolute measurement of the speed of light in 1862. Subsequent experiments performed by Marie Alfred Cornu in 1872–76 and by Albert A. Michelson in 1877–1931 used improved versions of the toothed wheel and rotating mirror experiments to make steadily more accurate estimates of the speed of light.

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