Armand-Hippolyte-Louis Fizeau

Armand-Hippolyte-Louis Fizeau was born in Paris in 1819 into a wealthy medical family. He began his studies intending to follow his father into medicine, but a severe stutter closed off any bedside clinical career — nineteenth-century medical practice demanded a confident verbal rapport with patients, and Fizeau could not produce it. He switched to physics at the École Polytechnique, where François Arago — Fresnel's former collaborator, then director of the Paris Observatory — became his mentor. His earliest work was photographic: with Léon Foucault, he made the first daguerreotype photograph of the Sun's surface (1845) and the first clear image of the Moon through a telescope — landmarks of early astrophotography.

His defining experiment came in 1849. Fizeau set up a rotating toothed wheel — 720 teeth around its rim — at his father's house in Suresnes on a hill outside Paris, and a mirror at Montmartre 8.63 km away. A light source behind the wheel was positioned so that light passed through a gap between two teeth, travelled the 8.63 km to the mirror, reflected, and returned through the wheel. By tuning the wheel's rotation speed, Fizeau could arrange for the returning light to pass through the *next* gap (wheel had rotated exactly one tooth-space during the round-trip — light path clear) or be blocked by the next tooth (wheel had rotated half a tooth-space — light path occluded). The transitions between transmission and extinction gave him a direct timing of the 17.26 km round-trip. The result: c ≈ 3.15 × 10⁸ m/s — the first terrestrial measurement of the speed of light, about 5% high of the modern 2.998 × 10⁸. Every previous c-measurement (Rømer 1676, Bradley 1729) had depended on astronomical observation of Jupiter's moons or stellar aberration. Fizeau had brought the measurement indoors.

Foucault followed in 1850 with a rotating-mirror method that improved the accuracy, and their rivalry became legend in Parisian physics — at one point the Academy invited both to present their methods in parallel sessions. Fizeau's other major contributions: he proposed the Doppler effect for light independently in 1848, five years after Doppler's acoustic derivation, so the effect is sometimes called the Doppler–Fizeau effect in French usage; he measured the partial drag of flowing water on the speed of light passing through it (1851), a result that became critical decades later as evidence against the luminiferous ether and in favour of special relativity; and he made early contributions to stellar interferometry that Michelson would fully develop. He received the Copley Medal in 1866. He died in Venteuil in 1896 at seventy-seven, long since the grand old man of French experimental physics.