Foucault , Jean Bernard Léon

(1819–1868) French physicist

Foucault, the son of a Parisian bookseller, originally intended to study medicine, but transferred his interest to physical science. In 1855 he became a physicist at the Paris Observatory.

His main work was on measurements of the speed of light. He helped Armand Fizeau in his toothed-wheel experiment and, in 1850, took over D.F.J. Arago's experiments on comparing the speed of light in air with that in water. The experiment was important for distinguishing between the wave and particle theories of light: the wave theory predicted that light should travel faster in air than in water; the particle theory predicted the opposite. In 1850, Foucault showed that the wave-theory prediction was correct. In 1862 he obtained the first accurate value for the speed of light using a rotating-mirror apparatus.

Foucault also worked on other topics. Thus he noted (1849) that a bright yellow line in the spectrum of sodium corresponded to a dark line in the Fraunhofer spectrum of the sun, although he failed to follow this up.

His most famous experiments began in 1850 and involved pendulums. While trying to construct an accurate timing device for his work on light, he noticed that a pendulum remained swinging in the same plane when he rotated the apparatus. He then used a pendulum to demonstrate the rotation of the Earth. Over a long period of time the plane in which a pendulum is swinging will appear to rotate. In fact the pendulum swings in a fixed plane relative to the fixed stars, and the Earth rotates ‘underneath’ it.

At the Earth's poles, the plane of the pendulum will make one full rotation every 24 hours; this period increases as the equator is approached. Foucault derived an equation relating the time of rotation to the latitude. He also gave public exhibitions of the effect, including one in which he suspended an iron ball of 28 kilograms (62 lbs) by steel wire 67 meters (222 ft) long from the dome of the Panthéon in Paris. Foucault also invented the gyroscope.