This entry contributed by Dana Romero
In 1849, French physicist Armand Fizeau Eric Weisstein's World of Biography developed a device known as the Fizeau wheel in order to measure the speed of light. This instrument consists of a rotating toothed wheel through which a beam of light is passed. The light is then reflected by a distant mirror, which reflects it back to the wheel. When the rotation speed is low, the light beam returns quickly enough so as to pass through the same opening through which it was transmitted. As the rotation speed increases, the light is blocked because the wheel has advanced one-half the distance between openings. Further increasing the speed, the wheel advances the entire distance between openings, and the beam again passes through. Knowing all the dimensions involved and the speeds at which the light beam passed or didn't pass, Fizeau could calculate the speed of light.
The apparatus was subsequently improved upon by Fizeau's contemporary Jean Foucault, Eric Weisstein's World of Biography who used a rotating polygonal mirror and a distant mirror. As the light beam returned from its path to the distant mirror, the rotating mirror had advanced slightly and the beam was reflected at a slight angle by a different face of the mirror. Knowing the deflection angle, the distance to the fixed mirror, and the speed of rotation, he calculated the speed of light to be 298,000,000 meters per second, very close to the currently accepted value.
