La Lettre de l'Académie des sciences n°37-38

37 38

La Lettre

Measuring the speed of light between the Observatoire de Paris and Montmartre, in 2005

foucault-texte.pdf

Hippolyte Fizeau

(1819-1896)

Sur les vitesses relatives de

la lumière dans l'air et dans

l'eau.

The doctoral thesis

Léon Foucault (1819-1868)

presented to the

Faculté des

sciences de Paris

, 1853.

of light in air and in a refractive medium such as water. One remembers, indeed, that, in Newton’s theory,

light accelerates when it penetrates a denser medium; on the contrary, according to the wave theory of

Huygens and Fresnel, light goes slower in the denser medium.

In 1849, Fizeau measured the speed of light between Montmartre and the Mont-

Valérien, near Paris. The website of the Observatoire de Paris presents a

remarkable explanation of these experiments, which were repeated in 2005

with a laser beam. In 1850, Foucault

improved the experimental set-up

and was able to realize the measu-

rement over a distance of only one

meter. He could then directly compare

the speed of light in air and water. The

thesis of the future member of the Academy

is only 35 pages long and it plainly concludes: "

Always,

light is delayed as it passes through the most refractive

medium. The final conclusion of this work thus consists

in declaring the system of emission incompatible with the

reality of the facts

." The last followers of the wave theory

then surrendered or passed – is it not a saying that new theo-

ries do prevail only when their detractors have passed?