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 cau ...

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. If a long and heavy pendulum suspended from the high roof above a circu ...

, 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 progra ...

. He also made an early measurement of the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

, discovered eddy currents, 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 (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...

Early years

The son of a publisher, Foucault was born in

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

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

physics Physics is the scientific study of matter, its Elementary particle, 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 whi ...

due to a

blood phobia Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is comp ...

. He first directed his attention to the improvement of

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

's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic

anatomy Anatomy () is the branch of morphology concerned with the study of the internal 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 scien ...

. With

Hippolyte Fizeau Armand Hippolyte Louis Fizeau (; 23 September 1819 – 18 September 1896) was a French physicist who, in 1849, measured the speed of light to within 5% accuracy. In 1851, he measured the speed of light in moving water in an experiment known as t ...

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

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

of the

sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, as compared with that of

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

in the

arc lamp An arc lamp or arc light is a lamp that produces light by an electric arc (also called a voltaic arc). The carbon arc light, which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the first decade of the 1800s, ...

, and of lime 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 that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

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

, 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 A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...

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 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. 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 Vincenzo Viviani (April 5, 1622 – September 22, 1703) was an Italian mathematician and scientist. He was a pupil of Torricelli and 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 (see Foucault's gyroscope experiment). In 1855, he received the

Copley Medal The Copley Medal is the most prestigious award of the Royal Society of the United Kingdom, conferred "for sustained, outstanding achievements in any field of science". The award alternates between the physical sciences or mathematics and the bio ...

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 cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

required for the rotation of a

copper Copper is a chemical element; it has symbol Cu (from Latin ) 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 pinkish-orang ...

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 eddy current 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" 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 patt ...

. 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 (; 6 February 1802 – 19 October 1875) was an English physicist and inventor best known for his contributions to the development of the Wheatstone bridge, originally invented by Samuel Hunter Christie, which is used to m ...

’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 ( ), formerly the Imperial Order of the Legion of Honour (), is the highest and most prestigious French national order of merit, both military and Civil society, civil. Currently consisting of five cl ...

. 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 f ...

'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 Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

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 sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

on 11 February 1868 in Paris and was buried in the

Montmartre Cemetery The Cemetery of 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 in Paris, after the Père Lachaise Cemet ...

. 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