Foucault's measurements of the speed of light

Experiments by Léon Foucault that used a rotating mirror to measure light’s speed and showed it travels more slowly in water than in air.

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Rotating-mirror method: An optical technique that measures light’s travel time from the angular shift of a rotating mirror. Foucault used this method to infer light’s speed from the mirror’s rotation during the beam’s journey.
Ole Rømer: A Danish astronomer whose observations of Jupiter’s moons provided an early estimate of light’s finite speed. His astronomical estimate preceded the laboratory measurements Foucault made on Earth.
Foucault speed-of-light measurement: A laboratory determination of light’s speed made with a rotating mirror and a measured optical path. Foucault’s apparatus demonstrated how precisely light’s speed could be measured without astronomical timing.
Fizeau's toothed-wheel experiment: A terrestrial method that estimates light’s speed from whether a rotating toothed wheel blocks a returning beam. Fizeau measured light with a wheel, while Foucault used a rotating mirror for greater precision.
Léon Foucault: A French physicist known for experimental work on light, mechanics, and Earth’s rotation. He designed and carried out the rotating-mirror measurements described here.
Hippolyte Fizeau: A French physicist who made the first successful terrestrial measurement of light’s speed in 1849. Fizeau’s toothed-wheel experiment set the experimental precedent Foucault improved.
Index of refraction: A dimensionless measure of how much light slows and bends when entering a material. The speed difference Foucault observed between air and water is expressed through their refractive indices.
Astronomical measurement of the speed of light: An estimate of light’s speed derived from timing celestial events across known astronomical distances. Foucault’s laboratory method avoided relying on planetary motion and astronomical timing.
Foucault's 1862 speed-of-light experiment: Foucault’s rotating-mirror experiment in which he measured light’s speed over a laboratory path. This specific experiment produced a value close to the modern speed of light.
Fizeau–Foucault experiments: Nineteenth-century experiments that measured light’s speed using toothed wheels and rotating mirrors. Foucault’s work belongs to this shift from astronomical estimates to terrestrial optical measurement.