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| Foucault's Most Famous Experiments |
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Léon Foucault attained world fame with two experiments. In 1850, with what is known as the rotating mirror method, he determined the velocity of the propagation of light. In his famous pendulum experiment in the Panthéon in Paris in 1851, he made the earth’s rotary motion visible to the general public.
Foucault and Fizeau (1819–1896) initially worked together in the field of photography. In 1845 they took the first picture of the sun. In addition, they examined the infrared spectrum and interference phenomena. After they agreed to go their separate ways on an amicable basis, each tried separately to determine the velocity of the propagation of light.
Sketch showing the principle behind Foucault’s method for measuring the velocity of light:
S: concave mirror
R: rotating mirror
G: glass plate
M: micrometer division
OO‘: image in the eyepiece.
Fizeau was successful with the toothed disk or wheel method in 1849, and Foucault with the rotating mirror method in 1851. Both supplied the proof required to confirm the wave theory, i.e. that light propagates more slowly in water than in air. In 1862 Foucault’s final measurements showed the velocity of light to be 298000 km/s.
In 1851 Foucault used a pendulum to demonstrate the earth’s rotary motion to the public, proving at the same time that the earth is not an inertial system. The "pendulum effect" is a result of the Coriolis force, a slow rotation of the pendulum’s plane of oscillation, observed with a long oscillation time, which is dependent on the geographic latitude of the location concerned.
Foucault used a pendulum weighing 28 kg on a wire 67 m long. This experiment had first been performed by Viviani (1672–1703) as early as 1661. In 1879 Kammerlingh Onnes quantitatively examined the error sources of Foucault’s pendulum and conducted precision measurements of the earth’s rotation with an error of less than 0.5%.
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