Editing Light (section)

==Historical theories about light, in chronological order==

===Classical Greece and Hellenism===
In the fifth century BC, [[Empedocles]] postulated that everything was composed of [[Classical element|four elements]]; fire, air, earth and water. He believed that goddess [[Aphrodite]] made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.<ref>{{Cite book |title=Fundamentals of Optical Engineering |last=Singh |first=S. |year=2009 |publisher=Discovery Publishing House |isbn=978-81-8356-436-6}}</ref>

In about 300 BC, [[Euclid]] wrote ''Optica'', in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. If the beam from the eye travels infinitely fast this is not a problem.<ref>{{Cite web |url=http://www-groups.dcs.st-and.ac.uk/history/HistTopics/Light_1.html |title=Light through the ages: Ancient Greece to Maxwell |last1=O'Connor |first1=J J |last2=Robertson |first2=E F |date=August 2002 |access-date=20 February 2017 |archive-date=19 March 2017 |archive-url=https://web.archive.org/web/20170319180859/http://www-groups.dcs.st-and.ac.uk/history/HistTopics/Light_1.html }}</ref>

In 55 BC, [[Lucretius]], a Roman who carried on the ideas of earlier Greek [[atomism|atomists]], wrote that "The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." (from ''On the nature of the Universe''). Despite being similar to later particle theories, Lucretius's views were not generally accepted. [[Ptolemy]] (c. second century) wrote about the [[refraction]] of light in his book ''Optics''.<ref>{{Cite book |title=Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary |author=Ptolemy and A. Mark Smith |publisher=Diane Publishing |year=1996 |page=23 |isbn=978-0-87169-862-9}}</ref>

===Classical India===
In [[Science and technology in ancient India|ancient India]], the [[Hindu]] schools of [[Samkhya]] and [[Vaisheshika]], from around the early centuries AD developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (''tanmatra'') out of which emerge the gross elements. The [[atomism|atomicity]] of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.<ref name="sifuae.com">{{cite web |url=http://www.sifuae.com/sif/wp-content/uploads/2015/04/Shastra-Pratibha-2015-Seniors-Booklet.pdf |title=Shastra Pratibha 2015 Seniors Booklet |website=Sifuae.com |access-date=29 August 2017 |archive-date=30 May 2015 |archive-url=https://web.archive.org/web/20150530101227/http://www.sifuae.com/sif/wp-content/uploads/2015/04/Shastra-Pratibha-2015-Seniors-Booklet.pdf }}</ref>
The ''[[Vishnu Purana]]'' refers to sunlight as "the seven rays of the sun".<ref name="sifuae.com"/>

The Indian [[Buddhist]]s, such as [[Dignāga]] in the fifth century and [[Dharmakirti]] in the seventh century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy.<ref name="sifuae.com"/>

===Descartes===
[[René Descartes]] (1596–1650) held that light was a [[Mechanism (philosophy)|mechanical]] property of the luminous body, rejecting the "forms" of [[Alhazen|Ibn al-Haytham]] and [[Witelo]] as well as the "species" of [[Roger Bacon#Legacy|Roger Bacon]], [[Robert Grosseteste]] and [[Johannes Kepler]].<ref name="Theories of light">''Theories of light, from Descartes to Newton'' A.I. Sabra CUP Archive,1981 p. 48 {{ISBN|978-0-521-28436-3}}</ref> In 1637 he published a theory of the [[refraction]] of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves.{{Citation needed|date=January 2010}} Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media.

Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes's theory of light is regarded as the start of modern physical optics.<ref name="Theories of light" />

===Particle theory===
{{Main|Corpuscular theory of light}}
[[File:PierreGassendi.jpg|thumb|200 px|[[Pierre Gassendi]]]]
[[Pierre Gassendi]] (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. [[Isaac Newton]] studied Gassendi's work at an early age and preferred his view to Descartes's theory of the ''plenum''. He stated in his ''Hypothesis of Light'' of 1675 that light was composed of [[Corpuscularianism|corpuscles]] (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the [[diffraction]] of light (which had been observed by [[Francesco Maria Grimaldi|Francesco Grimaldi]]) by allowing that a light particle could create a localised wave in the [[Aether (classical element)|aether]].

Newton's theory could be used to predict the [[Reflection (physics)|reflection]] of light, but could only explain [[refraction]] by incorrectly assuming that light accelerated upon entering a denser [[Medium (optics)|medium]] because the [[gravity|gravitational]] pull was greater. Newton published the final version of his theory in his ''[[Opticks]]'' of 1704. His reputation helped the [[particle theory of light]] to hold sway during the eighteenth century. The particle theory of light led [[Pierre-Simon Laplace]] to argue that a body could be so massive that light could not escape from it. In other words, it would become what is now called a [[black hole]]. Laplace withdrew his suggestion later, after a wave theory of light became firmly established as the model for light (as has been explained, neither a particle or wave theory is fully correct). A translation of Newton's essay on light appears in ''The large scale structure of space-time'', by [[Stephen Hawking]] and [[George F. R. Ellis]].

The fact that light could be [[polarized light|polarized]] was for the first time qualitatively explained by Newton using the particle theory. [[Étienne-Louis Malus]] in 1810 created a mathematical particle theory of polarization. [[Jean-Baptiste Biot]] in 1812 showed that this theory explained all known phenomena of light polarization. At that time the polarization was considered as the proof of the particle theory.

=== Wave theory ===
To explain the origin of [[colour]]s, [[Robert Hooke]] (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work ''[[Micrographia]]'' ("Observation IX"). In 1672 Hooke suggested that light's vibrations could be [[perpendicular]] to the direction of propagation. [[Christiaan Huygens]] (1629–1695) worked out a mathematical wave theory of light in 1678 and published it in his ''[[Treatise on Light]]'' in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the [[luminiferous aether]]. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.<ref>Fokko Jan Dijksterhuis, [https://books.google.com/books?id=cPFevyomPUIC Lenses and Waves: Christiaan Huygens and the Mathematical Science of Optics in the 17th Century], Kluwer Academic Publishers, 2004, {{ISBN|1-4020-2697-8}}</ref> Another supporter of the wave theory was [[Leonhard Euler]]. He argued in ''Nova theoria lucis et colorum'' (1746) that [[diffraction]] could more easily be explained by a wave theory.

[[File:Christiaan Huygens-painting.jpeg|thumb|upright=0.9|[[Christiaan Huygens]]]]
[[File:Young Diffraction.png|thumb|upright=0.9|[[Thomas Young (scientist)|Thomas Young]]'s sketch of a [[double-slit experiment]] showing [[diffraction]]. Young's experiments supported the theory that light consists of waves.]]

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by [[Thomas Young (scientist)|Thomas Young]]). Young showed by means of a [[double-slit experiment|diffraction experiment]] that light behaved as waves. He first publicly stated his "general law" of interference in January 1802, in his book ''A Syllabus of a Course of Lectures on Natural and Experimental Philosophy'':<ref>{{Cite journal |last=Mollon |first=J. D. |date=2002 |title=The Origins of the Concept of Interference |jstor=3066507 |journal=Philosophical Transactions: Mathematical, Physical and Engineering Sciences |volume=360 |issue=1794 |pages=807–819 |doi=10.1098/rsta.2001.0968 |pmid=12804280 |bibcode=2002RSPTA.360..807M |issn=1364-503X}}</ref><blockquote>But the general law, by which all these appearances are governed, may be very easily deduced from the interference of two coincident undulations, which either cooperate, or destroy each other, in the same manner as two musical notes produce an alternate intension and remission, in the beating of an imperfect unison.<ref>{{Cite book |last=Young |first=Thomas |title=A Syllabus of a Course of Lectures on Natural and Experimental Philosophy |publisher=W. Savage for The Royal Institution |year=1802 |location=London |publication-date=1802 |page=117}}</ref></blockquote>He also proposed that different colours were caused by different [[wavelength]]s of light and explained colour vision in terms of three-coloured receptors in the eye. 

In 1816 [[André-Marie Ampère]] gave [[Augustin-Jean Fresnel]] an idea that the polarization of light can be explained by the wave theory if light were a [[transverse wave]].<ref>James R. Hofmann, ''André-Marie Ampère: Enlightenment and Electrodynamics'', Cambridge University Press, 1996, p. 222.</ref> Later, Fresnel independently worked out his own wave theory of light and presented it to the [[Académie des Sciences]] in 1817. [[Siméon Denis Poisson]] added to Fresnel's mathematical work to produce a convincing argument in favor of the wave theory, helping to overturn Newton's corpuscular theory.{{dubious|date=June 2018}}<!-- [[Siméon Denis Poisson]] says he was an opponent of the theory --> By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained by the wave theory of light if and only if light was entirely transverse, with no longitudinal vibration whatsoever.{{Citation needed|date=June 2018}}

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical substance luminiferous aether proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the [[Michelson–Morley experiment]].

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the [[speed of light]] could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was [[Léon Foucault]], in 1850.<ref>{{Cite book |title=Understanding Physics |author1=David Cassidy |author2=Gerald Holton |author3=James Rutherford |publisher=Birkhäuser |year=2002 |isbn=978-0-387-98756-9 |url=https://books.google.com/books?id=rpQo7f9F1xUC&pg=PA382}}</ref> His result supported the wave theory, and the classical particle theory was finally abandoned (only to partly re-emerge in the twentieth century as [[photons]] in [[quantum mechanics|quantum theory]]).

===Electromagnetic theory===
{{Main|Electromagnetic radiation}}
[[File:Onde electromagnetique.svg|thumb|upright=1.8|A [[linear polarization|linearly polarized]] electromagnetic wave traveling along the z-axis, with E denoting the [[electric field]] and perpendicular B denoting [[magnetic field]]|400x400px]]

In 1845, [[Michael Faraday]] discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the [[magnetic field]] direction in the presence of a transparent [[dielectric]], an effect now known as [[Faraday rotation]].<ref name="LongairMalcolm">{{cite book |last=Longair |first=Malcolm |title=Theoretical Concepts in Physics |url=https://archive.org/details/theoreticalconce00mslo |url-access=limited |year=2003 |page=[https://archive.org/details/theoreticalconce00mslo/page/n106 87]}}</ref> This was the first evidence that light was related to [[electromagnetism]]. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.<ref name="LongairMalcolm" /> Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.<ref>{{Cite book|title=Understanding Physics|last=Cassidy|first=D|publisher=Springer Verlag New York|year=2002}}</ref>

Faraday's work inspired [[James Clerk Maxwell]] to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in ''On Physical Lines of Force''. In 1873, he published ''[[A Treatise on Electricity and Magnetism]]'', which contained a full mathematical description of the behavior of electric and magnetic fields, still known as [[Maxwell's equations]]. Soon after, [[Heinrich Hertz]] confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction and [[Wave interference|interference]]. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging and wireless communications.

In the quantum theory, photons are seen as [[wave packet]]s of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as [[spectral line]]s).

===Quantum theory===
In 1900 [[Max Planck]], attempting to explain [[black-body radiation]], suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "[[quantum|quanta]]" (from a Latin word for "how much"). In 1905, Albert Einstein used the idea of light quanta to explain the [[photoelectric effect]] and suggested that these light quanta had a "real" existence. In 1923 [[Arthur Holly Compton]] showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called [[Compton scattering]]) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926 [[Gilbert N. Lewis]] named these light quanta particles [[photon]]s.<ref>{{Cite book |url=https://archive.org/details/IntroductionToMolecularSpectroscopy |title=Introduction to Molecular Spectroscopy |last=Barrow |first=Gordon M. |publisher=McGraw-Hill |year=1962 |lccn=62-12478}}{{dead link|date=March 2025}}</ref>

Eventually [[quantum mechanics]] came to picture light as (in some sense) ''both'' a particle and a wave, and (in another sense) as a phenomenon which is ''neither'' a particle nor a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, under some approximations light can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles) and sometimes another macroscopic metaphor (waves).

As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both.

In 1924–1925, [[Satyendra Nath Bose]] showed that light followed different statistics from that of classical particles. With Einstein, they generalized this result for a whole set of integer spin particles called [[boson]]s (after Bose) that follow [[Bose–Einstein statistics]]. The photon is a massless boson of spin 1.

In 1927, [[Paul Dirac]] quantized the [[electromagnetic field]]. [[Pascual Jordan]] and [[Vladimir Fock]] generalized this process to treat many-body systems as excitations of quantum fields, a process with the misnomer of [[second quantization]]. And at the end of the 1940s a full theory of [[quantum electrodynamics]] was developed using quantum fields based on the works of [[Julian Schwinger]], [[Richard Feynman]], [[Freeman Dyson]], and [[Shinichiro Tomonaga]].

=== Quantum optics ===
{{main|Quantum optics}}
[[John R. Klauder]], [[George Sudarshan]], [[Roy J. Glauber]], and [[Leonard Mandel]] applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the [[Statistical mechanics|statistics]] of light (see [[degree of coherence]]). This led to the introduction of the [[coherent state]] as a concept which addressed variations between laser light, thermal light, exotic [[squeezed state]]s, etc. as it became understood that light cannot be fully described just referring to the [[electromagnetic field]]s describing the waves in the classical picture. In 1977, [[H. Jeff Kimble]] et al. demonstrated a single atom emitting one photon at a time, further compelling evidence that light consists of photons. Previously unknown quantum states of light with characteristics unlike classical states, such as [[Squeezed coherent state|squeezed light]] were subsequently discovered.

Development of short and [[Ultrashort pulse|ultrashort]] laser pulses—created by [[Q switching]] and [[modelocking]] techniques—opened the way to the study of what became known as ultrafast processes. Applications for solid state research (e.g. [[Raman spectroscopy]]) were found, and mechanical forces of light on matter were studied. The latter led to levitating and positioning clouds of atoms or even small biological samples in an [[optical trap]] or [[optical tweezers]] by laser beam. This, along with [[Doppler cooling]] and [[Sisyphus cooling]], was the crucial technology needed to achieve the celebrated [[Bose–Einstein condensation]].

Other remarkable results are the [[Bell test experiments|demonstration of quantum entanglement]], [[quantum teleportation]], and [[quantum logic gate]]s. The latter are of much interest in [[quantum information theory]], a subject which partly emerged from quantum optics, partly from theoretical [[computer science]].