Foucault, Jean Bernard Leon, foo KOH, zhahn behr NAR lay AWN (1819-1868), a French physicist, used a revolving mirror to measure the speed of light. Some types of measuring apparatus still use adaptations of his method. Foucault proved in 1850 that light travels more slowly in water than in air, and that the speed varies inversely with the index of refraction. He also made improvements in the mirrors of reflecting telescopes.