View source for Light

{{Short description|Electromagnetic radiation humans can see}}
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{{Redirect|Visible light}}
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[[File:Prism flat rainbow (cropped).jpg|thumb|upright=1.3|A triangular [[dispersive prism|prism]] [[dispersion (optics)|dispersing]] a beam of white light. The longer wavelengths (red) and the shorter wavelengths (green-blue) are separated.]]
{{Modern physics}}

'''Light''', '''visible light''', or '''visible radiation''' is [[electromagnetic radiation]] that can be [[visual perception|perceived]] by the [[human eye]].<ref>[[International Commission on Illumination|CIE]] (1987). [http://www.cie.co.at/publ/abst/17-4-89.html ''International Lighting Vocabulary''] {{Webarchive|url=https://web.archive.org/web/20100227034508/http://www.cie.co.at/publ/abst/17-4-89.html |date=27 February 2010 }}. Number 17.4. CIE, 4th ed.. {{ISBN|978-3-900734-07-7}}.<br />By the ''International Lighting Vocabulary'', the definition of ''light'' is: "Any radiation capable of causing a visual sensation directly."</ref><ref>{{Cite book |last=CIE JTC 8 |url=https://cie.co.at/publications/ilv-international-lighting-vocabulary-2nd-edition-0 |title=CIE S 017/E:2020 ILV: International Lighting Vocabulary, 2nd Edition |publisher=International Commission on Illumination (CIE) |language=en |chapter=light, <photometric> noun |doi=10.25039/S017.2020 |chapter-url=https://cie.co.at/eilvterm/17-21-013}}</ref> Visible light spans the [[visible spectrum]] and is usually defined as having [[wavelength]]s in the range of 400–700 [[nanometre]]s (nm), corresponding to [[frequency|frequencies]] of 750–420 [[terahertz (unit)|terahertz]]. The visible band sits adjacent to the [[infrared]] (with longer wavelengths and lower frequencies) and the [[ultraviolet]] (with shorter wavelengths and higher frequencies), called collectively ''[[optical radiation]]''.<ref name="Pal2001">{{cite book |last1=Pal |first1=G.K. |last2=Pal |first2=Pravati |title=Textbook of Practical Physiology |chapter-url=https://books.google.com/books?id=CcJvIiesqp8C&pg=PA387 |access-date=11 October 2013 |edition=1st |year=2001 |publisher=Orient Blackswan |location=Chennai |isbn=978-81-250-2021-9 |page=387 |chapter=chapter 52 |quote=The human eye has the ability to respond to all the wavelengths of light from 400–700 nm. This is called the visible part of the spectrum. |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031819/https://books.google.com/books?id=CcJvIiesqp8C&pg=PA387 |url-status=live }}</ref><ref name="BuserImbert1992">{{cite book |last1=Buser |first1=Pierre A. |last2=Imbert |first2=Michel |title=Vision |url=https://archive.org/details/vision0000buse |url-access=registration |access-date=11 October 2013 |year=1992 |publisher=MIT Press |isbn=978-0-262-02336-8 |page=[https://archive.org/details/vision0000buse/page/50 50] |quote=Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.}}</ref>

In [[physics]], the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not.<ref>{{Cite book |title=Camera lenses: from box camera to digital |author=Gregory Hallock Smith |publisher=SPIE Press |year=2006 |isbn=978-0-8194-6093-6 |page=4 |url=https://books.google.com/books?id=6mb0C0cFCEYC&pg=PA4 |access-date=15 November 2020 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031820/https://books.google.com/books?id=6mb0C0cFCEYC&pg=PA4 |url-status=live }}</ref><ref>{{Cite book |title=Comprehensive Physics XII |author=Narinder Kumar |publisher=Laxmi Publications |year=2008 |isbn=978-81-7008-592-8 |page=1416 |url=https://books.google.com/books?id=IryMtwHHngIC&pg=PA1416}}</ref> In this sense, [[gamma ray]]s, [[X-ray]]s, [[microwave]]s and [[radio wave]]s are also light. The primary properties of light are [[intensity (physics)|intensity]], propagation direction, frequency or wavelength [[spectrum]], and [[polarization (waves)|polarization]]. Its [[speed of light|speed in vacuum]], {{val|299792458|u=m/s}}, is one of the fundamental [[physical constant|constants]] of nature.<ref name="LeClerq">{{Cite book |last1=Uzan |first1=J-P |last2=Leclercq |first2=B |year=2008 |title=The Natural Laws of the Universe: Understanding Fundamental Constants |url=https://archive.org/details/the-natural-laws-of-the-universe-understanding-fundamental-constants |pages=43–44 |translator=Robert Mizon|isbn=978-0-387-73454-5|bibcode=2008nlu..book.....U |publisher=[[Springer-Praxis]], [[Internet Archive]]: 2020-06-14 AbdzexK uban|doi=10.1007/978-0-387-74081-2 |doi-broken-date=1 July 2025 }}</ref> All electromagnetic radiation exhibits some properties of both [[wave–particle duality|particles and waves]]. Single, massless elementary particles, or [[quantum|quanta]], of light called ''[[photon]]s'' can be detected with specialized equipment; phenomena like interference are described by waves. Most everyday interactions with light can be understood using [[geometrical optics]]; [[quantum optics]], is an important research area in [[modern physics]].

The main source of natural light on Earth is the [[Sun]]. Historically, another important source of light for humans has been [[fire]], from ancient campfires to modern [[kerosene lamp]]s. With the development of [[electric light]]s and [[history of electric power transmission|power systems]], electric lighting has effectively replaced firelight.

==Electromagnetic spectrum and visible light==
[[File:EM spectrum.svg|thumb|380px|The [[electromagnetic spectrum]], with the [[visible spectrum|visible portion]] highlighted. The bottom graph (Visible spectrum) is wavelength in units of nanometres (nm).]]
{{Main|Electromagnetic spectrum}}

Generally, [[electromagnetic radiation]] (EMR) is classified by wavelength into [[radio wave]]s, [[microwave]]s, [[infrared]], the [[visible spectrum]] that we perceive as light, [[ultraviolet]], [[X-ray]]s and [[gamma ray]]s. The designation "[[radiation]]" excludes [[Static electricity|static electric]], [[Magnetic field|magnetic]] and [[near and far field|near fields]].

The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

EMR in the visible light region consists of [[quantum|quanta]] (called [[photon]]s) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule [[retinal]] in the human retina, which change triggers the sensation of vision.

There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. [[Infrared sensing in snakes]] depends on a kind of natural [[thermal imaging]], in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it.

Above the frequency range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea with wavelengths shorter than 360 [[nanometer|nm]] and the internal lens at wavelengths shorter than 400&nbsp;nm. Furthermore, the [[rod cell|rods]] and [[cone cell|cones]] located in the [[retina]] of the human eye cannot detect the very short (shorter than 360&nbsp;nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses (such as insects and shrimp) are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light.

Various sources define visible light as narrowly as 420–680&nbsp;nm<ref>{{cite book |last=Laufer |first=Gabriel |chapter=Geometrical Optics |title=Introduction to Optics and Lasers in Engineering |chapter-url=https://books.google.com/books?id=4MxLPYMS5TUC&pg=PA11 |access-date=20 October 2013 |year= 1996 |isbn=978-0-521-45233-5 |page=11|doi=10.1017/CBO9781139174190.004 |bibcode=1996iole.book.....L }}</ref><ref name="Bradt2004">{{cite book |last=Bradt |first=Hale |title=Astronomy Methods: A Physical Approach to Astronomical Observations |url=https://books.google.com/books?id=hp7vyaGvhLMC&pg=PA26 |access-date=20 October 2013 |year=2004 |publisher=Cambridge University Press |isbn=978-0-521-53551-9 |page=26}}</ref> to as broadly as 380–800&nbsp;nm.<ref name="OhannesianStreeter2001">{{cite book |last1=Ohannesian |first1=Lena |last2=Streeter |first2=Anthony |title=Handbook of Pharmaceutical Analysis |url=https://books.google.com/books?id=DwPb4wgqseYC&pg=PA187 |access-date=20 October 2013 |year=2001 |publisher=CRC Press |isbn=978-0-8247-4194-5 |page=187}}</ref><ref name="AhluwaliaGoyal2000">{{cite book |last1=Ahluwalia |first1=V.K. |last2=Goyal |first2=Madhuri |title=A Textbook of Organic Chemistry |url=https://books.google.com/books?id=tJNJnn0M75MC&pg=PA110 |access-date=20 October 2013 |year= 2000 |publisher=Narosa |isbn=978-81-7319-159-6 |page=110}}</ref> Under ideal laboratory conditions, people can see infrared up to at least 1,050&nbsp;nm;<ref name="Sliney1976">{{cite journal |last1=Sliney |first1=David H. |last2=Wangemann |first2=Robert T. |last3=Franks |first3=James K. |last4=Wolbarsht |first4=Myron L. |year=1976 |title=Visual sensitivity of the eye to infrared laser radiation |journal=[[Journal of the Optical Society of America]] |volume=66 |issue=4 |pages=339–341 |doi=10.1364/JOSA.66.000339 |pmid=1262982 |quote=The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1,064 nm. A continuous 1,064 nm laser source appeared red, but a 1,060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina. |bibcode=1976JOSA...66..339S }}</ref> children and young adults may perceive ultraviolet wavelengths down to about 310–313&nbsp;nm.<ref name="LynchLivingston2001">{{cite book |last1=Lynch |first1=David K. |last2=Livingston |first2=William Charles |title=Color and Light in Nature |url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA231 |access-date=12 October 2013 |edition=2nd |year=2001 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-77504-5 |page=231 |quote=Limits of the eye's overall range of sensitivity extends from about 310 to 1,050 nanometers |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031821/https://books.google.com/books?id=4Abp5FdhskAC&pg=PA231 |url-status=live }}</ref><ref name="Dash2009">{{cite book |last1=Dash |first1=Madhab Chandra |last2=Dash |first2=Satya Prakash |title=Fundamentals of Ecology 3E |url=https://books.google.com/books?id=7mW4-us4Yg8C&pg=PA213 |access-date=18 October 2013 |year=2009 |publisher=Tata McGraw-Hill Education |isbn=978-1-259-08109-5 |page=213 |quote=Normally the human eye responds to light rays from 390 to 760 nm. This can be extended to a range of 310 to 1,050 nm under artificial conditions. |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031820/https://books.google.com/books?id=7mW4-us4Yg8C&pg=PA213 |url-status=live }}</ref><ref name="Saidman1933">{{cite journal |last1=Saidman |first1=Jean |date=15 May 1933 |title=Sur la visibilité de l'ultraviolet jusqu'à la longueur d'onde 3130 |trans-title=The visibility of the ultraviolet to the wave length of 3130 |journal=[[Comptes rendus de l'Académie des sciences]] |volume=196 |pages=1537–9 |language=fr |url=http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d |access-date=21 October 2013 |archive-date=24 October 2013 |archive-url=https://web.archive.org/web/20131024092515/http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d |url-status=live }}</ref>

Plant growth is also affected by the colour spectrum of light, a process known as [[photomorphogenesis]].

== Speed of light ==
{{Main|Speed of light}}
[[File:Rocca dell'Abisso, Fondachelli Fantina, Sicilia.JPG|thumb|Beam of sun light inside the cavity of Rocca ill'Abissu at [[Fondachelli-Fantina]], Sicily]]
The speed of light in [[vacuum]] is defined to be exactly {{val|299792458|u=m/s}} (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.

Different [[physicist]]s have attempted to measure the speed of light throughout history. [[Galileo Galilei|Galileo]] attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by [[Ole Rømer]], a Danish physicist, in 1676. Using a [[telescope]], Rømer observed the motions of [[Jupiter]] and one of its [[natural satellite|moons]], [[Io (moon)|Io]]. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.<ref>{{cite journal |url=http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.ss%2F1009212817 |title=Scientific Method, Statistical Method and the Speed of Light |journal=Statistical Science |year=2000 |volume=15 |pages=254–278 |issue=3 |doi=10.1214/ss/1009212817 |mr=1847825 |last1=Oldford |first1=R. W |last2=MacKay |first2=R. J |doi-access=free |access-date=21 August 2008 |archive-date=24 March 2017 |archive-url=https://web.archive.org/web/20170324201543/http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.ss%2F1009212817 |url-status=live }}</ref> However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of {{val|227000000|u=m/s}}.

Another more accurate measurement of the speed of light was performed in Europe by [[Hippolyte Fizeau]] in 1849.<ref name=EB1911>{{cite EB1911 |wstitle=Light |volume=16 |page=624 |first=Simon |last=Newcomb}}</ref> Fizeau directed a beam of light at a mirror several kilometers away. A rotating [[cog wheel]] was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel and the rate of rotation, Fizeau was able to calculate the speed of light as {{val|313000000|u=m/s}}.

[[Léon Foucault]] carried out an experiment which used rotating mirrors to obtain a value of {{val|298000000|u=m/s}}<ref name=EB1911/> in 1862. [[Albert A. Michelson]] conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from [[Mount Wilson (California)|Mount Wilson]] to [[Mount San Antonio]] in California. The precise measurements yielded a speed of {{val|299796000|u=m/s}}.<ref>{{cite journal |last=Michelson |first=A.A. |title=Measurements of the velocity of light between Mount Wilson and Mount San Antonio |journal=Astrophysical Journal |date=January 1927 |volume=65 |page=1 |doi=10.1086/143021 |bibcode=1927ApJ....65....1M}}</ref>

The effective velocity of light in various transparent substances containing ordinary [[matter]], is less than in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists were said to bring light to a "complete standstill" by passing it through a [[Bose–Einstein condensate]] of the element [[rubidium]], one team at [[Harvard University]] and the [[Rowland Institute for Science]] in Cambridge, Massachusetts and the other at the [[Harvard–Smithsonian Center for Astrophysics]], also in Cambridge.<ref>{{cite web |author=Harvard News Office |url=http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html |title=Harvard Gazette: Researchers now able to stop, restart light |publisher=News.harvard.edu |date=24 January 2001 |access-date=8 November 2011 |archive-url=https://web.archive.org/web/20111028041346/http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html |archive-date=28 October 2011 }}</ref> However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped", it had ceased to be light.

==Optics==
{{Main|Optics}}
[[File:Linear visible spectrum.svg|center|upright=2]]

The study of light and the interaction of light and [[matter]] is termed ''[[optics]]''. Optics has different forms appropriate to different circumstances. [[Geometrical optics]], appropriate for understanding things like eyes, lenses, cameras, [[fiber optics]], and mirrors, works well when the wavelength of light is small in comparison to the objects it interacts with. [[Physical optics]] incorporates wave properties and is needed understand diffraction and interference. [[Quantum optics]] applies when studying individual photons interacting with matter.<ref>{{Cite book |last=Hecht |first=Eugene |title=Optics |date=1998 |publisher=Addison-Wesley |isbn=978-0-201-83887-9 |edition=3. |location=Reading, Mass. Harlow}}</ref>{{rp|33}}

=== Surface scattering ===

A [[Transparency (optics)|transparent]] object allows light to [[Transmittance|transmit]] or pass through. Conversely, an [[Opacity (optics)|opaque]] object does not allow light to transmit through and instead [[Reflection (physics)|reflecting]] or [[Absorbance|absorbing]] the light it receives. Most objects do not reflect or transmit light [[Specular reflection|specularly]] and to some degree [[Scattering|scatters]] the incoming light, which is called [[Gloss (optics)|glossiness]]. Surface scattering is caused by the [[surface roughness]] of the reflecting surfaces, and internal scattering is caused by the difference of [[refractive index]] between the particles and [[Transmission medium|medium]] inside the object. Like transparent objects, [[translucent]] objects allow light to transmit through, but translucent objects also scatter certain wavelength of light via internal scattering.<ref>{{Cite book |last=Berns |first=Roy S. |url= |title=Billmeyer and Saltzman's Principles of Color Technology |publisher=[[Wiley (publisher)|Wiley]] |others=Fred W. Billmeyer, Max Saltzman |year=2019 |isbn=978-1-119-36668-3 |edition=4th |location=Hoboken, NJ |oclc=1080250734}}</ref>

===Refraction===
{{Main|Refraction}}
[[File:Optical refraction at water surface.jpg|thumb|Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.]]

Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by [[Snell's Law]]:

<math display=block>n_1\sin\theta_1 = n_2\sin\theta_2</math>

where {{math|''θ''<sub>1</sub>}} is the angle between the ray and the surface [[Normal (geometry)|normal]] in the first medium, {{math|''θ''<sub>2</sub>}} is the angle between the ray and the surface normal in the second medium and {{math|''n''<sub>1</sub>}} and {{math|''n''<sub>2</sub>}} are the [[Index of refraction|indices of refraction]], {{math|1=''n'' = 1}} in a [[vacuum]] and {{math|''n'' > 1}} in a [[Transparency and translucency|transparent]] [[Chemical substance|substance]].

When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not [[orthogonality|orthogonal]] (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as [[refraction]].

The refractive quality of [[lens (optics)|lenses]] is frequently used to manipulate light in order to change the apparent size of images. [[Magnifying glass]]es, [[Glasses|spectacles]], [[contact lens]]es, [[microscope]]s and [[refracting telescope]]s are all examples of this manipulation.

==Light sources== <!-- This section is linked from [[Source]] -->
{{Redirect|Lightsource|the solar energy developer named Lightsource|Lightsource Renewable Energy|a particle accelerator used to generate X-rays|Synchrotron light source}}
{{Further|List of light sources}}

There are many sources of light. A body at a given temperature emits a characteristic spectrum of [[black-body radiation]]. A simple thermal source is [[sunlight]], the radiation emitted by the [[chromosphere]] of the [[Sun]] at around {{cvt|6000|K|C F|lk=on}}. Solar radiation peaks in the visible region of the [[electromagnetic spectrum]] when plotted in wavelength units,<ref>{{cite web |url=http://thulescientific.com/LYNCH%20%26%20Soffer%20OPN%201999.pdf |title=Spectrum and the Color Sensitivity of the Eye |website=Thulescientific.com |access-date=29 August 2017 |archive-date=5 July 2010 |archive-url=https://web.archive.org/web/20100705070506/http://thulescientific.com/LYNCH%20%26%20Soffer%20OPN%201999.pdf |url-status=live}}</ref> and roughly 44% of the radiation that reaches the ground is visible.<ref>{{cite web |url=http://rredc.nrel.gov/solar/spectra/am1.5/ |title=Reference Solar Spectral Irradiance: Air Mass 1.5 |access-date=12 November 2009 |archive-date=12 May 2019 |archive-url=https://web.archive.org/web/20190512190812/https://rredc.nrel.gov/solar//spectra/am1.5/ |url-status=live}}</ref> Another example is [[incandescent light bulb]]s, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in [[fire|flames]], but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.

The peak of the black-body spectrum is in the deep infrared, at about 10 [[micrometre]] wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is heated to "red hot" or "white hot". Blue-white [[thermal emission]] is not often seen, except in stars (the commonly seen pure-blue colour in a [[natural gas|gas]] flame or a [[welder]]'s torch is in fact due to molecular emission, notably by CH radicals emitting a wavelength band around 425&nbsp;nm and is not seen in stars or pure thermal radiation).

Atoms emit and absorb light at characteristic energies. This produces "[[emission line]]s" in the spectrum of each atom. [[Emission (electromagnetic radiation)|Emission]] can be [[spontaneous emission|spontaneous]], as in [[light-emitting diode]]s, [[gas discharge]] lamps (such as [[neon lamp]]s and [[neon sign]]s, [[mercury-vapor lamp]]s, etc.) and flames (light from the hot gas itself—so, for example, [[sodium]] in a gas flame emits characteristic yellow light). Emission can also be [[stimulated emission|stimulated]], as in a [[laser]] or a microwave [[maser]].

Deceleration of a free charged particle, such as an [[electron]], can produce visible radiation: [[cyclotron radiation]], [[synchrotron radiation]] and [[bremsstrahlung]] radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible [[Cherenkov radiation]]. Certain chemicals produce visible radiation by [[chemoluminescence]]. In living things, this process is called [[bioluminescence]]. For example, [[firefly|fireflies]] produce light by this means and boats moving through water can disturb plankton which produce a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as [[fluorescence]]. Some substances emit light slowly after excitation by more energetic radiation. This is known as [[phosphorescence]]. Phosphorescent materials can also be excited by bombarding them with subatomic particles. [[Cathodoluminescence]] is one example. This mechanism is used in [[cathode-ray tube]] [[television set]]s and [[computer monitor]]s.

[[File:Colorful artificial lighting at night.jpg|thumb|250px|[[Hong Kong]] illuminated by colourful artificial [[lighting]]]]

Certain other mechanisms can produce light:
* [[Electroluminescence]]
* [[Scintillation (physics)|Scintillation]]
* [[Sonoluminescence]]
* [[Triboluminescence]]

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
* Particle–[[antiparticle]] annihilation
* [[Radioactive decay]]

==Measurement==
{{Main|Photometry (optics)|Radiometry}}

Light is measured with two main alternative sets of units: [[radiometry]] consists of measurements of light power at all wavelengths, while [[photometry (optics)|photometry]] measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify [[Illumination (lighting)]] intended for human use.

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The [[cone cell]]s in the human eye are of three types which respond differently across the visible spectrum and the cumulative response peaks at a wavelength of around 555&nbsp;nm. Therefore, two sources of light which produce the same intensity (W/m<sup>2</sup>) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw [[power (physics)|power]] by a quantity called [[luminous efficacy]] and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a [[photocell]] sensor does not necessarily correspond to what is perceived by the human eye and without filters which may be costly, photocells and [[charge-coupled device]]s (CCD) tend to respond to some [[infrared]], [[ultraviolet]] or both.

==Light pressure==
{{Main|Radiation pressure}}
Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by [[Maxwell's equations]], but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by ''[[speed of light|c]]'', the speed of light.{{Spaces}} Due to the magnitude of ''c'', the effect of light pressure is negligible for everyday objects.{{Spaces}} For example, a one-milliwatt [[laser pointer]] exerts a force of about 3.3 [[newton (unit)|piconewtons]] on the object being illuminated; thus, one could lift a [[penny (United States coin)|U.S. penny]] with laser pointers, but doing so would require about 30&nbsp;billion 1-mW laser pointers.<ref>{{cite journal |last=Tang |first=Hong |title=May The Force of Light Be With You |journal=IEEE Spectrum |date=1 October 2009 |volume=46 |issue=10 |pages=46–51 |doi=10.1109/MSPEC.2009.5268000|s2cid=7928030 }}</ref>{{Spaces}} However, in [[nanometre]]-scale applications such as [[nanoelectromechanical systems]] (NEMS), the effect of light pressure is more significant and exploiting light pressure to drive NEMS mechanisms and to flip nanometre-scale physical switches in integrated circuits is an active area of research.<ref>See, for example, [http://www.eng.yale.edu/tanglab/research.htm nano-opto-mechanical systems research at Yale University] {{Webarchive|url=https://web.archive.org/web/20100625042036/http://www.eng.yale.edu/tanglab/research.htm |date=25 June 2010 }}.</ref> At larger scales, light pressure can cause [[asteroid]]s to spin faster,<ref>{{cite web |url=http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/ |title=Asteroids Get Spun By the Sun |first=Kathy A. |last=Svitil |website=Discover Magazine |date=5 February 2004 |access-date=8 May 2007 |archive-date=9 October 2012 |archive-url=https://web.archive.org/web/20121009045611/http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/ |url-status=live }}</ref> acting on their irregular shapes as on the vanes of a [[windmill]].{{Spaces}} The possibility of making [[solar sail]]s that would accelerate spaceships in space is also under investigation.<ref>{{cite web |url=http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html |title=Solar Sails Could Send Spacecraft 'Sailing' Through Space |website=NASA |date=31 August 2004 |access-date=30 May 2008 |archive-date=21 October 2012 |archive-url=https://web.archive.org/web/20121021035846/http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html |url-status=live }}</ref><ref>{{cite web |url=http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html |title=NASA team successfully deploys two solar sail systems |website=NASA |date=9 August 2004 |access-date=30 May 2008 |archive-date=14 June 2012 |archive-url=https://web.archive.org/web/20120614013757/http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html |url-status=live }}</ref>

Although the motion of the [[Crookes radiometer]] was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.<ref>{{cite journal |author-link=Pyotr Lebedev |first=P. |last=Lebedew |title=Untersuchungen über die Druckkräfte des Lichtes |journal=Annalen der Physik |volume=6 |issue=11 |pages=433–458 |year=1901 |doi=10.1002/andp.19013111102 |bibcode=1901AnP...311..433L |url=https://zenodo.org/record/1424005 |access-date=29 July 2022 |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606184159/https://zenodo.org/record/1424005 |url-status=live }}</ref> This should not be confused with the [[Nichols radiometer]], in which the (slight) motion caused by torque (though not enough for full rotation against friction) ''is'' directly caused by light pressure.<ref>{{cite journal |last1=Nichols |first1=E.F |last2=Hull |first2=G.F. |year=1903 |url=https://books.google.com/books?id=8n8OAAAAIAAJ&q=torsion+balance+radiation&pg=RA5-PA327 |title=The Pressure due to Radiation |journal=The Astrophysical Journal |volume=17 |pages=315–351 |issue=5 |bibcode=1903ApJ....17..315N |doi=10.1086/141035 |access-date=15 November 2020 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008031820/https://books.google.com/books?id=8n8OAAAAIAAJ&q=torsion+balance+radiation&pg=RA5-PA327 |url-status=live |doi-access=free }}</ref>
As a consequence of light pressure, [[Albert Einstein|Einstein]] in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter.<ref>{{cite book |last=Einstein, A. |chapter=Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung |trans-chapter=On the development of our views concerning the nature and constitution of radiation |title=The Collected Papers of Albert Einstein |volume=2 |year=1989 |orig-date=1909 |publisher=Princeton University Press |location=Princeton, New Jersey |page=391}}</ref> He wrote, "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."

Usually light momentum is aligned with its direction of motion. However, for example in [[evanescent wave]]s momentum is transverse to direction of propagation.<ref>{{Cite journal|last1=Antognozzi|first1=M.|last2=Bermingham|first2=C. R.|last3=Harniman|first3=R. L.|last4=Simpson|first4=S.|last5=Senior|first5=J.|last6=Hayward|first6=R.|last7=Hoerber|first7=H.|last8=Dennis|first8=M. R.|last9=Bekshaev|first9=A. Y.|date=August 2016|title=Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever|journal=Nature Physics|volume=12|issue=8|pages=731–735|doi=10.1038/nphys3732|issn=1745-2473|arxiv=1506.04248|bibcode=2016NatPh..12..731A|s2cid=52226942}}</ref>

==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 water waves showing [[diffraction]]<ref name=Rothman>
{{cite book
 |last1=Rothman
 |first1=T.
 |author1-link=Tony Rothman
 |year=2003
 |title=Everything's Relative and Other Fables in Science and Technology
 |publisher=[[John Wiley & Sons]]
 |isbn=978-0-471-20257-8
 |url-access=registration
 |url=https://archive.org/details/everythingsrelat0000roth
 }}</ref>]]

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 numerous [[diffraction]] experiments that light behaved as waves.<ref name=Whittaker>{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 2: The modern theories, 1900 - 1926 |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |place=New York }}</ref>{{rp|101}} 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]] challenged Fresnel's model, claiming that it predicted a bright spot in the shadow behind a circular obstacle contrary to common sense. [[Dominique-François-Jean Arago]] created an experiment that showed the [[Arago spot|bright spot]]: Poisson's challenge became new evidence for the wave theory.<ref name=Whittaker/>{{rp|109}} In 1818, Young wrote to Arago suggesting that light must be transverse waves, not the longitudinal waves characteristic of sound. Fresnel took up the idea and was able to show via mathematical methods that polarization could be explained by a transverse wave theory of light with no longitudinal vibration.<ref name=Whittaker/>{{rp|115}}

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 of light|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 (physics)|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]].

==Use for light on Earth==
[[Sunlight]] provides the [[energy]] that [[green plants]] use to create [[sugars]] mostly in the form of [[starch]]es, which release energy into the living things that digest them. This process of [[photosynthesis]] provides virtually all the energy used by living things. Some species of animals generate their own light, a process called [[bioluminescence]]. For example, [[fireflies]] use light to locate mates and [[vampire squid]] use it to hide themselves from prey.

==See also==
{{Portal|Physics|Science}}
{{cols|colwidth=26em}}
* [[Ballistic photon]]
* [[Colour temperature]]
* [[Fermat's principle]]
* [[Huygens' principle]]
* ''[[Journal of Luminescence]]''
* [[Light beam]] – in particular about light beams visible from the side
* [[Light Fantastic (TV series)|''Light Fantastic'' (TV series)]]
* [[Light mill]]
* [[List of light sources]]
* ''[[Luminescence: The Journal of Biological and Chemical Luminescence]]''
* [[Spectroscopy]]
{{colend}}

==Notes==
{{Reflist|group=nb}}

==References==
{{Reflist}}

==External links==
* {{Commons category-inline|Light}}
* {{Wiktionary inline}}
* {{Wikiquote-inline}}

{{Radiation}}
{{Colour topics}}
{{Authority control}}

[[Category:Light| ]]
[[Category:Electromagnetic radiation]]