The son of a publisher, Foucault was born inParis on 18 September 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour ofphysics due to ablood phobia.[1] He first directed his attention to the improvement ofLouis Daguerre's photographic processes. For three years he was experimental assistant toAlfred Donné (1801–1878) in his course of lectures on microscopicanatomy.[2]
WithHippolyte Fizeau he carried out a series of investigations on the intensity of thelight of thesun, as compared with that ofcarbon in thearc lamp, and oflime in the flame of the oxyhydrogen blowpipe; on the interference ofinfraredradiation, and of light rays differing greatly in lengths of path; and on the chromaticpolarization of light.[2]
In 1849, Foucault experimentally demonstrated thatabsorption andemission lines appearing at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source.[3][4]
In 1850, he did an experiment using arotating mirror to measure the speed of light; it was viewed as "driving the last nail in the coffin" ofNewton'scorpuscular theory of light when it showed that light travels more slowly through water than through air.[5]In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (diurnal motion). This experimental setup had been used byVincenzo Viviani but became well known to the public by Foucault's work. Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of thePanthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulums" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named[6]) thegyroscope as a conceptually simpler experimental proof (seeFoucault's gyroscope experiment). In 1855, he received theCopley Medal of theRoyal Society for his 'very remarkable experimental researches'. Earlier in the same year he was madephysicien (physicist) at the imperial observatory at Paris.
In September 1855 he discovered that theforce required for the rotation of acopper disc becomes greater when it is made to rotate with its rim between the poles of amagnet, the disc at the same time becoming heated by theeddy current or "Foucault currents" induced in the metal.
Diagram of a variant of Foucault's speed of light experiment where a modern laser is the source of light
In 1857 Foucault invented the polarizer which bears his name,[7] and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape.[8][9] The so-called "Foucault knife-edge test" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in itsfigure. Prior to Foucault's publication of his findings, the testing of reflecting telescope mirrors was a "hit or miss" proposition.
Foucault's knife edge test determines the shape of a mirror by finding the focal lengths of its areas, commonly called zones and measured from the mirror center. In the test, light from a point source is focused onto the center of curvature of the mirror and reflected back to a knife edge. The test enables the tester to quantify the conic section of the mirror, thereby allowing the tester to validate the actual shape of the mirror, which is necessary to obtain optimal performance of the optical system. The Foucault test is in use to this date, most notably by amateur and smaller commercial telescope makers as it is inexpensive and uses simple, easily made equipment.
WithCharles Wheatstone’s revolving mirror he, in 1862, determined the speed of light to be 298,000km/s – 10,000 km/s less than that obtained by previous experimenters[2] and only 0.6% in error of thecurrently accepted value.
In 1862 Foucault was made a member of theBureau des Longitudes and an officer of theLegion of Honour. He became a member of theRoyal Society of London in 1864, and member of the mechanical section of the Institute a year later. In 1865 he published his papers on a modification ofJames Watt'scentrifugal governor; he had for some time been experimenting with a view to making its period of revolution constant and developing a new apparatus for regulating the electric light. Foucault showed how, by the deposition of a transparently thin film ofsilver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye. His chief scientific papers are to be found in theComptes Rendus, 1847–1869.[10] Near his death he returned to Roman Catholicism that he previously abandoned.[11]
^Brand, John C. D. (1995).Lines of Light: The Sources of Dispersive Spectroscopy. Luxembourg: Gordon and Breach Publishers. pp. 60–62.ISBN978-2884491624.
^William Tobin (2003).The Life and Science of Léon Foucault: The Man Who Proved the Earth Rotates. Cambridge University Press. p. 272.ISBN9780521808552.
^W. Tobin, The Life and Science of Léon Foucault, Cambridge University Press (2003).
Amir D. Aczel,Pendulum: Léon Foucault and the Triumph of Science, Washington Square Press, 2003,ISBN0-7434-6478-8
Umberto Eco,Foucault's Pendulum (trans. William Weaver). Secker & Warburg, 1989.
William Tobin,Perfecting the Modern Reflector. Sky & Telescope, October 1987.
William Tobin, Evolution of the Foucault-Secretan Reflecting Telescope.Journal of Astronomical History and Heritage, 19, 106–184pdf & 361–362pdf, 2016.
William Tobin,Léon Foucault. Scientific American, July 1998.
William Tobin,The Life and Science of Léon Foucault: The Man who Proved the Earth Rotates. Cambridge University Press, 2003.ISBN0-521-80855-3
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