Jean Bernard Léon Foucault (18.09.1819-11.02.1868) was a French experimental physicist and a member of the Paris Academy of Sciences (1865). He was born in Paris. From 1855 he was a physicist at the Paris Observatory, and from 1862 a member of the Bureau of Longitudes.
His research was in the fields of optics, mechanics and electromagnetism. In 1850 he developed a method of measuring the speed of light with the help of a rotating mirror (the Foucault method) and measured the speed of light in air and in water. According to Foucault, the speed of light in water was 3/4 of its speed in air, which finally confirmed the wave theory of light. In 1862 Foucault returned to measuring the speed of light in air and obtained for it the value (298000±500) km/s. With a pendulum (the Foucault pendulum) he proved experimentally in 1851 the rotation of the Earth about its axis. In 1852 he invented the gyroscope, which found wide application in technology. In 1855 he detected the heating of solid metal bodies by induced currents (Foucault currents) and proposed a way of reducing it.
He invented the first automatic regulator of light for the arc lamp. He was the first to develop a precise method of making mirrors for large reflectors and proposed (1857) using, instead of metal mirrors, lighter and cheaper glass ones, coated with a thin film of silver (the telescope with a silvered glass mirror). He was the first to establish (1849) the connection between absorption lines and emission lines.
He was a Fellow of the Royal Society, a member of the Berlin Academy of Sciences and a corresponding member of the St Petersburg Academy of Sciences (1860). Copley Medal.