Léon Foucault

{{for|the comic book character Jeanne Foucault|Finesse (character)}}

{{Infobox scientist

| name = Léon Foucault

| image = Foucault portre crop.jpg

| image_size = 225px

| caption = Photograph by Bertall & Cie

| birth_name = Jean Bernard Léon Foucault

| birth_date = 18 September 1819

| birth_place = Paris, France

| death_date = {{Death date and age|1868|2|11|1819|9|18|df=yes}}

| death_place = Paris, France

| resting_place = Montmartre Cemetery

| field = Physics

| work_institution = Paris Observatory

| alma_mater = University of Paris

| doctoral_advisor =

| doctoral_students =

| known_for = Foucault knife-edge test
Foucault pendulum
Foucault's currents
Foucault's gyroscope
{{no wrap|Foucault's measurements of the speed of light}}
Glan–Foucault prism
Diurnal motion
Gyroscope
Catadioptric systems
Silvering process

| author_abbreviation_bot =

| author_abbreviation_zoo =

| prizes = Copley Medal {{small|(1855)}}
FRS {{small|(1864)}}

| footnotes =

}}

Jean Bernard Léon Foucault ({{IPAc-en|UK|ʒ|ɒ̃|_|ˈ|b|ɛər|n|ɑːr|_|ˌ|l|eɪ|ɒ̃|_|ˈ|f|uː|k|oʊ}}, {{IPAc-en|US|ˌ|ʒ|ɒ̃|_|b|ɛər|ˈ|n|ɑːr|_|l|eɪ|ˌ|ɒ̃|_|f|uː|ˈ|k|oʊ}}; {{IPA|fr|ʒɑ̃ bɛʁnaʁ leɔ̃ fuko|lang}}; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measurement of the speed of light, discovered eddy currents, and is credited with naming the gyroscope.

Early years

The son of a publisher, Foucault was born in Paris on 18 September 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of physics due to a blood phobia.{{CathEncy|url=http://www.newadvent.org/cathen/06156c.htm|title=Jean-Bertrand-Léon Foucault}} He first directed his attention to the improvement of Louis Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy.{{EB1911|inline=y|wstitle=Foucault, Jean Bernard Léon|volume=10|page=734}}

With Hippolyte Fizeau he carried out a series of investigations on the intensity of the light of the sun, as compared with that of carbon in the arc lamp, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of infrared radiation, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.

In 1849, Foucault experimentally demonstrated that absorption and emission 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.{{cite book |last1=Brand |first1=John C. D. |title=Lines of Light: The Sources of Dispersive Spectroscopy |date=1995 |publisher=Gordon and Breach Publishers |location=Luxembourg |isbn=978-2884491624 |pages=60–62}}See:

{{cite journal |last1=Foucault |first1=L. |title=Lumière électrique |journal=L'Institut, Journal Universel des Sciences |date=7 February 1849 |volume=17 |issue=788 |pages=44–46 |url=https://books.google.com/books?id=HggFrG2xd50C&pg=RA1-PA44 |trans-title=Electric light |language=fr}}
{{cite journal |last1=Foucault |first1=L. |title=Lumière électrique |journal=Société Philomatique de Paris. Extraits des Procès-Verbaux de Séances. |date=1849 |volume=13 |pages=16–20 |url=https://www.biodiversitylibrary.org/item/98244#page/106/mode/1up |trans-title=Electric light |language=fr}}

Middle years

Image:Foucault_pendulum.ogv

In 1850, he did an experiment using a rotating mirror to measure the speed of light; it was viewed as "driving the last nail in the coffin" of Newton's corpuscular theory of light when it showed that light travels more slowly through water than through air.{{cite book | title = Understanding Physics |author1=David Cassidy |author2=Gerald Holton |author3=James Rutherford | publisher = Birkhäuser | year = 2002 | isbn = 0-387-98756-8 | url = https://books.google.com/books?id=rpQo7f9F1xUC&q=Foucault+speed-of-light+wave+theory&pg=PA382}}

In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (diurnal motion). This experimental setup had been used by Vincenzo 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 the Panthéon, 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{{cite book|title=The Oxford English Dictionary|edition=2nd|year=1989|volume=VI|page=985 |url=https://www.oed.com/view/Entry/82917 |access-date=11 March 2023}}) the gyroscope as a conceptually simpler experimental proof (see Foucault's gyroscope experiment). In 1855, he received the Copley Medal of the Royal Society for his 'very remarkable experimental researches'. Earlier in the same year he was made physicien (physicist) at the imperial observatory at Paris.

In September 1855 he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current or "Foucault currents" induced in the metal.

Image:Speed of light (foucault).PNG

In 1857 Foucault invented the polarizer which bears his name,Léon Foucault (August 17, 1857) [https://books.google.com/books?id=wplDAQAAIAAJ&pg=PA238 "Nouveau polariseur en spath d'Island. Expérience de fluorescence"] (New polarizer made of Icelandic spar. Fluorescence experiment.), Comptes rendus, vol. 45, pages 238–241. English translation: Léon Foucault (1857) [https://books.google.com/books?id=BhFDAQAAIAAJ&pg=PA552 "On a new polarizer of Iceland spar. Experiment on fluorescence.,"] The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, series 4, vol. 14, pages 552–555. and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape.L. Foucault (1858) [https://books.google.com/books?id=v3MbAAAAMAAJ&pg=PA958 "Description des procédés employes pour reconnaitre la configuration des surfaces optiques"] (Description of the methods used to recognize the configuration of optical surfaces), Comptes rendus ... , vol. 47, pages 958–959.L. Foucault (1859) [https://books.google.com/books?id=m6Y3AQAAIAAJ&pg=PA197 "Mémoire sur la construction des télescopes en verre argenté"] (Memoir on the construction of reflecting telescopes), Annales de l'Observatoire impériale de Paris, vol. 5, pages 197–237. The so-called "Foucault knife-edge test" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure. 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.

With Charles Wheatstone’s revolving mirror he, in 1862, determined the speed of light to be 298,000 km/s – 10,000 km/s less than that obtained by previous experimenters and only 0.6% in error of the currently accepted value.

Later years

In 1862 Foucault was made a member of the Bureau des Longitudes and an officer of the Legion of Honour. He became a member of the Royal 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 of James Watt's centrifugal 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 of silver 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 the Comptes Rendus, 1847–1869.{{sfn|Chisholm|1911}} Near his death he returned to Roman Catholicism that he previously abandoned.{{cite book|title=The Life and Science of Léon Foucault: The Man Who Proved the Earth Rotates|year=2003|publisher=Cambridge University Press|isbn=9780521808552|author=William Tobin|page=272}}

Death and afterwards

Image:Foucault jean bernard leon grave montmartre paris 01.jpg]]

Foucault died of what was probably a rapidly developing case of multiple sclerosisW. Tobin, The Life and Science of Léon Foucault, Cambridge University Press (2003). on 11 February 1868 in Paris and was buried in the Montmartre Cemetery.

The asteroid 5668 Foucault was named for him.{{Cite book |last=Schmadel |first=Lutz D.|author-link=Lutz D. Schmadel|author2=International Astronomical Union |title=Dictionary of minor planet names |year=2003 |publisher=Springer-Verlag |location=Berlin; New York |isbn=978-3-540-00238-3 |page=480|url=https://books.google.com/books?id=KWrB1jPCa8AC&pg=PA480|access-date=9 September 2011}} His is one of the 72 names inscribed on the Eiffel Tower.

Bibliography

Collected Works:

Recueil des travaux scientifiques de Léon Foucault [http://docnum.u-strasbg.fr/u?/coll12,54772 Volume One], [http://docnum.u-strasbg.fr/u?/coll12,55394 Volume Two], 1878. (Université de Strasbourg) or [https://archive.org/details/recueildestrava00lissgoog Internet Archive] (US)
[http://web2.bium.univ-paris5.fr/livanc/?cote=00576&do=livre Donné & Foucault Atlas of medical micrographs] 1845

Léon Foucault

Early years

Middle years

Later years

Death and afterwards

Bibliography

See also

References

Further reading

Léon&nbsp;Foucault

Early years

Middle years

Later years

Death and afterwards

Bibliography

See also

References

Further reading

Léon Foucault