Physicist, b. 18 September 1819 (Paris, France), d. 11 February 1868 (Paris).
Foucault's education was in medicine, but his interest in physics turned him to experimentation. He developed a technique to measure the velocity of light and established that light travels slower in water than in air. His measurements were accurate to within 1 percent of today's figure.
Today Foucault's name is remembered for his demonstration experiment of the rotation of the Earth. A pendulum that swings accurately in a perfect plane (i.e. not on an elliptical path) will maintain its movement in that plane relative to space, while the Earth rotates under it. If the pendulum is placed at one of the poles its plane of movement will perform an apparent full rotation in 24 hours.
At the equator the pendulum will be swung around with the Earth's movement but maintain its plane of movement relative to the Earth, so no rotation of the pendulum's movement is observed.
Based on the mathematical derivation of the effect published in 1835 by Gustave-Gaspard Coriolis, Foucault predicted that at places between the poles and the equator the pendulum's plane of movement will perform one complete rotation in a period given by 24 hours divided by the sine of the latitude. In Paris this resulted in a period of about 30 hours, as was observed. Foucault published a description of his experiment in 1851.
Portrait: public domain (Wikipedia)