Armand Fizeau

Armand-Hippolyte-Louis Fizeau (1819-1896) is reputed to be the first person to employ Christian Doppler's work in acoustics (the Doppler Effect) to light and made what is considered to be the first accurate measurement of the speed of light.

From the hilltop at Montmarte in Paris, Fizeau sent a beam of light eight kilometres long to a mirror on the hill at Suresnes which returned back through a slit in a spinning wheel. Knowing the speed of the rotation of the wheel, he was able to calculate the velocity of light within 5% of modern estimations.

Fizeau also provided empirical data from his observations that light travels slower through water than through air.

Fizeau's observations and those of Foucault's were a significant piece of direct observation that was to support the wave-theory of light as opposed to the corpuscular theory that had been posited by Sir Isaac Newton. Newton's theory predicted that light would travel faster though water than air.

Léon Foucault (1819-1868), a colleague of Fizeau, was to later calculate the speed of light to within 1% of modern estimations.