Apparatus for measuring the speed of light

François Arago’s work on the speed of light was pursued by Léon Foucault at the Paris Observatory. His experiment consisted in passing a beam of light through a micrometric target composed of vertical lines 1/10th of a millimetre apart. The rays were then reflected in a mirror turning at 400 revolutions/minute, then, successively, in four static mirrors. Between its departure and return, the beam has covered 20 metres and the rotating mirror has turned slightly. By measuring this angle the speed of light can be determined, which in 1862 Foucault gave as 298,000 kilometres/second, very close to the currently agreed speed of 299,792 kilometres/second.