Light travels at 299,792.458 km/second in space and at only minutely reduced speed in air. It is thus not a trivial task to measure its speed in a simple experiment.
French scientists of the 19th century devised several methods to arrive at a reasonably accurate value for the speed of light. TWo methods, one devised by Armand-Hippolyte-Louis Fizeau (1819 - 1896) in 1849, the other by Jean-Bernard-Léon Foucault, are outlined below.
In Fizeau's experiment of 1849 a permanent light source is set up at L. Its beam is focussed through a lens C and directed through a half-transparent mirror P that sends it to the mirror M, from where it is reflected and seen by the observer at A. The beam passes through a toothed wheel mounted with its axle at R . As the wheel rotates the beam is converted into a series of light pulses that are reflected by the mirror M but only seen by the observer at A if their path is not obstructed by a tooth as they return. By slowly adjusting the speed of rotation of the wheel until complete darkness is obtained at A it is possible to determine how fast the pulse of light travels between P and M.
Fizeau performed the experiment between Suresnes and Montmartre in Paris over a distance of 8,633 m and obtained a value of 315,300 km/second, within about 4 percent of today's accepted value. Alfred Cornu repeated the experiment over a distance of 23 km and obtained 300,030 km/second, well within 1 percent.
In 1850 Fouocault independently devised a different method that allowed him to determine the speed of light in the laboratory. A light beam is reflected by a primary mirror towards a secondary mirror that sends the light back to the primary mirror. Under stationary conditions the light beam is thus returned to its source. Foucault set the primary mirror into rapid rotation, so that the light beam ends up at a position slightly shifted from the place from where it was emitted. He could then determine the speed of light by measuring the length of the light path, the rotation rate of the mirror and the angle by which the returned beam is moved from its source.
Foucault's light path was about 20 m; his measured speed of light was within 1 percent of today's accepted value.