Léon Foucault

Léon Foucault (1819–1868) was a French physicist whose precision experiments measured light’s speed, demonstrated Earth’s rotation with a pendulum, and advanced gyroscope design.

Foucault pendulum: A long pendulum whose changing swing plane demonstrates Earth’s rotation. Foucault’s 1851 Paris demonstration made this the clearest public proof of his experimental skill.
Coriolis effect: The apparent deflection of moving objects in a rotating reference frame. It helps explain how Earth’s rotation changes a pendulum’s swing direction relative to the ground.
Armand Fizeau: A French physicist who made early terrestrial measurements of the speed of light. Fizeau’s toothed-wheel method preceded Foucault’s rotating-mirror measurement.
Speed of light measurement: The experimental determination of how fast light propagates. Foucault’s work established a more accurate laboratory route to measuring this fundamental constant.
Speed of light: The rate at which electromagnetic radiation propagates through space or a material. Foucault measured it in the laboratory and showed light travels more slowly in water than in air.
Angular momentum: A conserved physical quantity describing rotational motion. Conservation of angular momentum underlies both gyroscopic stability and Foucault’s rotation experiments.
François Arago: A French astronomer and physicist who studied optics, magnetism, and astronomy. Arago encouraged Foucault’s optical investigations and helped bring his experiments to scientific attention.
Michelson–Morley experiment: A 1887 experiment that tested whether Earth’s motion could be detected through the luminiferous ether. Its rotating optical apparatus developed a tradition of precision measurement associated with Foucault’s work.
Foucault's rotating mirror experiment: An optical experiment that uses a rotating mirror to measure the speed of light. This was Foucault’s method for obtaining a terrestrial measurement of light’s speed.
Interference (wave propagation): The superposition of waves that can reinforce or cancel one another. Foucault’s optical measurements depended on resolving small changes in light paths.

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