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Foucault, (Jean Bernard) Léon

light speed earth pendulum

[fookoh] (1819–68) French physicist: measured the speed of light; and demonstrated rotation of the Earth.

Foucault was the son of an impoverished bookseller; he began to study medicine but was revolted by the sight of blood. He was attracted to physics by discovery of photography, and then became editor of the Journal de Débats (1845) and a physicist at the Paris Observatory (1855). A gifted experimentalist with great originality and instinct, he died of paralysis aged 48, having been elected to the Académie des Sciences (1865) and the Royal Society of London (1864).

Foucault collaborated with on the toothed-wheel experiment which first measured the speed of light terrestrially. In 1850 he took over experimental equipment and first measured the speed of light in water, showing that it was slower than in air. This was important evidence in favour of the wave theory of light and contrary to the prediction of particle theory of light. Foucault then constructed a rotating mirror experiment for measuring the speed of light and used it to obtain a more accurate value (1862).

The simplicity and imaginativeness of Foucault’s measurement in 1850 of the rotation of the Earth by a swinging pendulum (Foucault’s pendulum) make it an outstanding achievement. The effect occurs because the Earth rotates, leaving the plane of the swinging pendulum fixed with respect to the stars. In 1852 Foucault demonstrated this with a 67 m pendulum with a 28 kg ball hung from the dome of the Panthéon in Paris. The experiment was carried out there with the help of Napoleon III, before an admiring crowd who watched a needle attached to the ball inscribe a mark in sand; the mark moved as the Earth rotated about the plane of the pendulum’s swing. This was the first direct (ie non-deductive) demonstration of the Earth’s rotation.

Other valuable work by Foucault included invention of the gyroscope in 1852, and improvements to reflecting telescopes. He discovered the yellow (sodium D) lines in emission spectra corresponding to the dark lines seen in absorption spectra by ; the value of this, however, was only realized later .

[back] Foster, Gloria (1936–2003)

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about 7 years ago

ok like how long did it take to build foucault's pendulum