Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...

best known for his demonstration of the

Foucault pendulum The Foucault pendulum or Foucault's pendulum is a simple device named after French physicist Léon Foucault, conceived as an experiment to demonstrate the Earth's rotation. A long and heavy pendulum suspended from the high roof above a circular ...

, a device demonstrating the effect of

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...

. He also made an early measurement of the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

, discovered

eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a mag ...

s, and is credited with naming the

gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rota ...

Early years

The son of a publisher, Foucault was born in

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...

on 18 September 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

due to a

blood phobia Blood phobia (also known as hemophobia or hematophobia in American English and haemophobia or haematophobia in British English) is an extreme irrational fear of blood, a type of specific phobia. Severe cases of this fear can cause physical reacti ...

. He first directed his attention to the improvement of

Louis Daguerre Louis-Jacques-Mandé Daguerre ( , ; 18 November 1787 – 10 July 1851) was a French artist and photographer, recognized for his invention of the eponymous daguerreotype process of photography. He became known as one of the fathers of photog ...

's photographic processes. For three years he was experimental assistant to

Alfred Donné Alfred may refer to: Arts and entertainment *''Alfred J. Kwak'', Dutch-German-Japanese anime television series * ''Alfred'' (Arne opera), a 1740 masque by Thomas Arne * ''Alfred'' (Dvořák), an 1870 opera by Antonín Dvořák *"Alfred (Interlu ...

(1801–1878) in his course of lectures on microscopic

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...

. With

Hippolyte Fizeau Armand Hippolyte Louis Fizeau FRS FRSE MIF (; 23 September 181918 September 1896) was a French physicist, best known for measuring the speed of light in the namesake Fizeau experiment. Biography Fizeau was born in Paris to Louis and Beatrice Fi ...

he carried out a series of investigations on the intensity of the

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

of the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, as compared with that of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

in the arc lamp, and of

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

in the flame of the oxyhydrogen blowpipe; on the interference of

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

, 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.

Middle years

In 1850, he did an experiment using the

Fizeau–Foucault apparatus The Fizeau–Foucault apparatus is either of two types of instrument historically used to measure the speed of light. The conflation of the two instrument types arises in part because Hippolyte Fizeau and Léon Foucault had originally been friends ...

to measure the speed of light; it came to be known as the Foucault–Fizeau experiment, and was viewed as "driving the last nail in the coffin" of Newton's

corpuscular theory of light In optics, the corpuscular theory of light states that light is made up of small discrete particles called " corpuscles" (little particles) which travel in a straight line with a finite velocity and possess impetus. This was based on an alternate ...

when it showed that light travels more slowly through water than through air. In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (

diurnal motion Diurnal motion (, ) is an astronomical term referring to the apparent motion of celestial objects (e.g. the Sun and stars) around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's ro ...

). This experimental setup had been used by

Vincenzo Viviani Vincenzo Viviani (April 5, 1622 – September 22, 1703) was an Italian mathematician and scientist. He was a pupil of Torricelli and a disciple of Galileo.Panthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and "''

s''" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the

as a conceptually simpler experimental proof. In 1855, he received the

Copley Medal The Copley Medal is an award given by the Royal Society, for "outstanding achievements in research in any branch of science". It alternates between the physical sciences or mathematics and the biological sciences. Given every year, the medal is t ...

of the

Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...

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 In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...

required for the rotation of a

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

disc becomes greater when it is made to rotate with its rim between the poles of a

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

, the disc at the same time becoming heated by the

or "Foucault currents" induced in the metal. In 1857 Foucault invented the polarizer which bears his name, and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape. The so-called "

Foucault knife-edge test The Foucault knife-edge test is an optical test to accurately measure the shape of concave curved mirrors. It is commonly used by amateur telescope makers for figuring primary mirrors in reflecting telescopes. It uses a relatively simple, inexpens ...

" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its

figure Figure may refer to: General *A shape, drawing, depiction, or geometric configuration *Figure (wood), wood appearance *Figure (music), distinguished from musical motif *Noise figure, in telecommunication *Dance figure, an elementary dance pattern ...

. 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 Sir Charles Wheatstone FRS FRSE DCL LLD (6 February 1802 – 19 October 1875), was an English scientist and inventor of many scientific breakthroughs of the Victorian era, including the English concertina, the stereoscope (a device for di ...

’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 The National Order of the Legion of Honour (french: Ordre national de la Légion d'honneur), formerly the Royal Order of the Legion of Honour ('), is the highest French order of merit, both military and civil. Established in 1802 by Napoleon, ...

. He became a member of the

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 James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fun ...

centrifugal governor A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional cont ...

; 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 Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

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. Near his death he returned to Roman Catholicism that he previously abandoned.

Death and afterwards

Foucault died of what was probably a rapidly developing case of

multiple sclerosis Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...

on 11 February 1868 in Paris and was buried in the

Montmartre Cemetery The Cemetery of Montmartre (french: link=no, Cimetière de Montmartre) is a cemetery in the 18th arrondissement of Paris, France, that dates to the early 19th century. Officially known as the Cimetière du Nord, it is the third largest necropolis ...

. The asteroid 5668 Foucault was named for him. His is one of the 72 names inscribed on the Eiffel Tower.

Bibliography

Collected Works: * ''Recueil des travaux scientifiques de Léon Foucault'
Volume OneVolume Two
1878. (Université de Strasbourg) o
Internet Archive
(US)
Donné & Foucault ''Atlas'' of medical micrographs
1845

Early years

Middle years

Later years

Death and afterwards

Bibliography

See also

References

Further reading