Foucault, Jean Bernard Leon (1819-1868), a French physicist. Foucault was trained for a medical career before turning to physics. In 1852 he built a device he named a gyroscope, to show how the earth turns on its axis. Earlier, in an experiment with a pendulum at the Pantheon in Paris, he had demonstrated a way to provide visible proof of the earth's rotation. Today, pendulums that demonstrate the earth's rotation are called Foucault pendulums.
Foucault was one of the first physicists to measure the speed of light accurately, and he showed that light travels more slowly in water than in air. In 1855 he was appointed physicist at the Paris Observatory. He invented the Foucault prism for analyzing light in 1857.