Armand Fizeau

Armand Fizeau (1819-1896), French physicist who determined the speed of light by experiment. Fizeau was born in Paris on September 23, 1819. His father was a professor at the Paris Faculty of Medicine. Fizeau gave up his medical studies because of poor health and instead took up the physical sciences, in particular the study of optics.

Fizeau improved Daguerre’s photographic process shortly after it was announced in 1839, and also developed a photo-etching technique. In 1845, Fizeau and another young physicist, Léon Foucault, may well have produced the first clear photographs of the Sun’s surface. Their collaboration ended with a quarrel in 1850, but their six years of joint experiments were fruitful in elucidating the wave nature of light. With a spectroscope and birefringent crystal (one that exhibits double refraction) they demonstrated that light waves were more complex than sound waves in that they were transverse and not longitudinal. They showed that the infrared (“calorific”) rays of the spectrum produced interference fringes the same as visible light, except that these fringes appeared as alternating bands of hot and cold. In 1848, independent of Christian Doppler, Fizeau showed that the shift in the wavelength of light coming from a star was a measure of the star’s velocity. He made the first reasonably accurate determination of the speed of light (1849) by means of a toothed-wheel apparatus and a light beam projected over a distance of about 8 km (5 mi). He failed to detect the ether that had been postulated to occupy all space as the carrier of light waves. In the early 20th century this failure would lead to the end of the ether theory.

Fizeau became a member of the French Academy of Sciences in 1860, and was appointed Superintendent of Physics at the École Polytechnique, Paris, in 1863. He died on September 18, 1896, in Nanteuil-le-Haudouin, France.