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| {{Redirect|Visible light|light that cannot be seen with human eye|Electromagnetic radiation|other uses|Light (disambiguation)|and|Visible light (disambiguation)}}
| | The name of the writer is Numbers. Managing individuals is what I do and the wage has been really fulfilling. North Dakota is exactly where me and my husband live. What I love performing is performing ceramics but I haven't produced a dime with it.<br><br>my web page; [http://kamtsw.org/?mid=p5_14&Document_srl=154713&document_srl=690146 at home std testing] |
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| [[File:The sun1.jpg|thumb|250px|The [[Sun]] is Earth's primary source of light. About 44% of the sun's electromagnetic radiation that reaches the ground is in the visible light range.]]
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| '''Visible light''' (commonly referred to simply as '''light''') is [[electromagnetic radiation]] that is [[Visual perception|visible]] to the [[human eye]], and is responsible for the sense of [[Visual perception|sight]].<ref>[[International Commission on Illumination|CIE]] (1987). [http://www.cie.co.at/publ/abst/17-4-89.html ''International Lighting Vocabulary'']. Number 17.4. CIE, 4th edition. 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> Visible light is usually defined as having a [[wavelength]] in the range of 400 [[nanometre]]s (nm), or 400×10<sup>−9</sup> m, to 700 nanometres – between the [[infrared light|infrared]], with longer wavelengths and the [[ultraviolet light|ultraviolet]], with shorter wavelengths.<ref name="Pal2001">{{cite book|last1=Pal|first1=G. K.|last2=Pal|first2=Pravati|title=Textbook of Practical Physiology|url=http://books.google.com/books?id=CcJvIiesqp8C&pg=PA387|accessdate=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.}}</ref><ref name="BuserImbert1992">{{cite book|last1=Buser|first1=Pierre A.|last2=Imbert|first2=Michel|title=Vision|url=http://books.google.com/books?id=NSZvt8Ld2-8C&pg=PA50|accessdate=11 October 2013|year=1992|publisher=MIT Press|isbn=978-0-262-02336-8|page=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> These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances. Various sources define visible light as narrowly as 420 to 680<ref>{{cite book|last=Laufer|first=Gabriel|title=Introduction to Optics and Lasers in Engineering|url=http://books.google.com/books?id=4MxLPYMS5TUC&pg=PA11|accessdate=20 October 2013|date=13 July 1996|publisher=Cambridge University Press|isbn=978-0-521-45233-5|page=11}}</ref><ref name="Bradt2004">{{cite book|last=Bradt|first=Hale|title=Astronomy Methods: A Physical Approach to Astronomical Observations|url=http://books.google.com/books?id=hp7vyaGvhLMC&pg=PA26|accessdate=20 October 2013|year=2004|publisher=Cambridge University Press|isbn=978-0-521-53551-9|page=26}}</ref> to as broadly as 380 to 800 nm.<ref name="OhannesianStreeter2001">{{cite book|last1=Ohannesian|first1=Lena|last2=Streeter|first2=Anthony|title=Handbook of Pharmaceutical Analysis|url=http://books.google.com/books?id=DwPb4wgqseYC&pg=PA187|accessdate=20 October 2013|date=9 November 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=http://books.google.com/books?id=tJNJnn0M75MC&pg=PA110|accessdate=20 October 2013|date=1 January 2000|publisher=Narosa|isbn=978-81-7319-159-6|page=110}}</ref> Under ideal laboratory conditions, people can see infrared up to at least 1050 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 | url =http://www.opticsinfobase.org/josa/abstract.cfm?uri=josa-66-4-339 | 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 1064 nm. A continuous 1064 nm laser source appeared red, but a 1060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina. |subscription=yes}}</ref> children and young adults ultraviolet down to about 310 to 313 nm.<ref name="LynchLivingston2001">{{cite book|last1=Lynch|first1=David K.|last2=Livingston|first2=William Charles|title=Color and Light in Nature|url=http://books.google.com/books?id=4Abp5FdhskAC&pg=PA231|accessdate=12 October 2013|edition=2nd|year=2001|publisher=Cambridge University Press|location=Cambridge, UK|isbn=978-0-521-77504-5|page=231|quote=Limits of the eye's overall range of sensitivity extends from about 310 to 1050 nanometers}}</ref><ref name="Dash2009">{{cite book|last1=Dash|first1=Madhab Chandra|last2=Dash|first2=Satya Prakash|title=Fundamentals Of Ecology 3E|url=http://books.google.com/books?id=7mW4-us4Yg8C&pg=PA213|accessdate=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 articial conditions.}}</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=French | url =http://visualiseur.bnf.fr/ark:/12148/bpt6k3148d}}</ref>
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| Primary properties of visible light are [[intensity (physics)|intensity]], propagation direction, [[frequency]] or [[wavelength]] [[spectrum]], and [[polarization (waves)|polarisation]], while its [[speed of light|speed]] in a vacuum, 299,792,458 meters per second, is one of the fundamental [[Mathematical constant|constants]] of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.
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| In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called [[photon]]s, and exhibits properties of both [[wave]]s and [[Particle physics|particles]]. This property is referred to as the [[wave–particle duality]]. The study of light, known as [[optics]], is an important research area in modern physics.
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| In [[physics]], the term ''light'' sometimes refers 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 = http://books.google.com/?id=6mb0C0cFCEYC&pg=PA4}}</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 = http://books.google.com/?id=IryMtwHHngIC&pg=PA1416}}</ref> This article focuses on visible light. See the [[electromagnetic radiation]] article for the general term.
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| ==Speed of light==
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| {{Main|Speed of light}}
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| The speed of light in a [[free space|vacuum]] is defined to be exactly 299,792,458 [[Metre per second|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.
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| 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|moon]]s, [[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/1009212817|title=Scientific Method, Statistical Method and the Speed of Light|journal=Statistical Science|year=2000|volume=15|pages=254–278|issue=3}}</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 227,000,000 m/s.
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| Another, more accurate, measurement of the speed of light was performed in Europe by [[Hippolyte Fizeau]] in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating [[Cog-wheel|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 313,000,000 m/s.
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| [[Léon Foucault]] used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. [[Albert Abraham Michelson|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 [[mirror]]s to measure the [[time]] it took light to make a round trip from [[Mount Wilson (California)|Mt. Wilson]] to [[Mt. San Antonio]] in [[California]]. The precise measurements yielded a speed of 299,796,000 m/s.
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| 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. However, the slowing process in matter is thought to result not from actual slowing of particles of light, but rather from their absorption and re-emission from charged particles in matter.
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| As an extreme example of the nature of light-slowing in matter, two independent teams of physicists were able 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, Mass., 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=2001-01-24 |accessdate=2011-11-08}}</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.
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| ==Electromagnetic spectrum and visible light==
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| {{Main|Electromagnetic spectrum}}
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| [[File:EM spectrum.svg|thumb|380px|right|[[Electromagnetic spectrum]] with light highlighted]]
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| Generally, EM radiation, or EMR (the designation 'radiation' excludes static electric and magnetic and [[near and far field|near fields]]) is classified by wavelength into [[radio]], [[microwave]], [[infrared]], the '''visible region''' that we perceive as light, [[ultraviolet]], [[X-ray]]s and [[gamma rays]].
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| The behaviour 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 behaviour depends on the amount of energy per quantum it carries.
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| EMR in the visible light region consists of quanta (called [[photon]]s) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which lead 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.
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| 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.
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| Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the [[rod cell|rods]] and [[cones]] located in the [[retina]] of the human eye cannot detect the very short (below 360 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.
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| [[File:Linear visible spectrum.svg|center|800px]]
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| ==Optics==
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| {{Main|Optics}}
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| The study of light and the interaction of light and [[matter]] is termed [[optics]]. The observation and study of [[optical phenomenon|optical phenomena]] such as [[rainbow]]s and the [[Aurora (astronomy)|aurora borealis]] offer many clues as to the nature of light.
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| ===Refraction===
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| {{Main|Refraction}}
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| [[Image:Refraction-with-soda-straw.jpg|thumb|250 px|An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.]]
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| [[File:Cloud in the sunlight.jpg|thumb|250px|A [[cloud]] illuminated by [[sunlight]]]]
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| 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]]:
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| :<math>n_1\sin\theta_1 = n_2\sin\theta_2\ .</math>
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| where <math>\theta_1</math> is the angle between the ray and the [[Normal (geometry)|surface normal]] in the first medium, <math>\theta_2</math> is the angle between the ray and the surface normal in the second medium, and n<sub>1</sub> and n<sub>2</sub> are the [[index of refraction|indices of refraction]], ''n'' = 1 in a [[vacuum]] and ''n'' > 1 in a [[Transparency and translucency|transparent]] [[Chemical substance|substance]].
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| 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]].
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| The refractive quality of [[lens (optics)|lens]]es 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.
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| ==Light sources== <!-- This section is linked from [[Source]] -->
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| {{Further|List of light sources}}
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| There are many sources of light. The most common light sources are thermal: 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 6,000 [[Kelvin]] peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units <ref>http://thulescientific.com/LYNCH%20&%20Soffer%20OPN%201999.pdf</ref> and roughly 44% of sunlight energy 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|accessdate=2009-11-12}}</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 blackbody spectrum is in the deep infrared, at about 10 [[micrometer]] 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 [[heat]]ed 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 [[Welding|welder's]] torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm, and is not seen in stars or pure thermal radiation).
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| 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]].
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| 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]].
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| 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.
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| 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]].
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| 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.
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| [[File:Night yamagata city 2.jpg|thumb|250px|A [[city]] illuminated by [[lighting|artificial lighting]]]]
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| Certain other mechanisms can produce light:
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| * [[Bioluminescence]]
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| * [[Cherenkov radiation]]
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| * [[Electroluminescence]]
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| * [[Scintillation (physics)|Scintillation]]
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| * [[Sonoluminescence]]
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| * [[triboluminescence]]
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| When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
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| * Particle–[[antiparticle]] annihilation
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| * [[Radioactive decay]]
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| ==Units and measures==
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| {{Main|Photometry (optics)|Radiometry}}
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| 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 standardised model of human brightness perception. Photometry is useful, for example, to quantify [[Illumination (lighting)]] intended for human use. The SI units for both systems are summarised in the following tables.
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| {{SI radiometry units|1|self|nb}}<!-- parameter 1 for table number, parameter 2 for compare destination page, parameter 3 for reference group -->
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| {{SI light units|2|self|nb}}<!-- parameter 1 for table number, parameter 2 for compare destination page, parameter 3 for reference group -->
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| 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 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.
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| ==Light pressure==
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| {{Main|Radiation pressure}}
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| 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.{{nbsp}} Due to the magnitude of ''c'', the effect of light pressure is negligible for everyday objects.{{nbsp}} For example, a one-[[watt|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.{{nbsp}}S. penny]] with laser pointers, but doing so would require about 30 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}}</ref>{{nbsp}} However, in [[nanometer]]-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 nanometer-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].</ref>
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| 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
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| | author = Kathy A. | work = Discover Magazine | date = 2004-02-05}}</ref> acting on their irregular shapes as on the vanes of a [[windmill]].{{nbsp}} 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 | work = [[NASA]] | date = 2004-08-31}}</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 | work = NASA | date = 2004-08-09}}</ref>
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| 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>P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).</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|last=Nichols|first=E.F|last2=Hull|first2=G.F.|year=1903|url=http://books.google.com/books?id=8n8OAAAAIAAJ&pg=RA5-PA327&dq=torsion+balance+radiation|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 }}</ref>
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| ==Historical theories about light, in chronological order==
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| ===Classical Greece and Hellenism===
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| {{Refimprove section|date=May 2011}}
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| In the fifth century BC, [[Empedocles]] postulated that everything was composed of [[Classical element|four elements]]; fire, air, earth and water. He believed that [[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.
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| 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. Of course if the beam from the eye travels infinitely fast this is not a problem.
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| In 55 BC, [[Lucretius]], a Roman who carried on the ideas of earlier Greek [[atomism|atomists]], wrote:
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| "''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.''" – ''On the nature of the Universe''
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| Despite being similar to later particle theories, Lucretius's views were not generally accepted.
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| [[Ptolemy]] (c. 2nd 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 = 0-87169-862-5}}</ref>
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| ===Classical India===
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| {{Refimprove section|date=May 2011}}
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| In [[Science and technology in ancient India|ancient India]], the [[Hindu]] schools of [[Samkhya]] and [[Vaisheshika]], from around the early centuries CE 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.
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| On the other hand, the Vaisheshika school gives an [[atomic theory]] of the physical world on the non-atomic ground of [[Aether (classical element)|ether]], space and time. (See ''[[Atomism#Indian atomism|Indian atomism]]''.) The basic atoms are those of earth (''prthivi''), water (''pani''), fire (''agni''), and air (''vayu'') Light rays are taken to be a stream of high velocity of ''tejas'' (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the ''tejas'' atoms.{{Citation needed|date=January 2012}}
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| The ''[[Vishnu Purana]]'' refers to [[sunlight]] as "the seven rays of the sun".{{Citation needed|date=January 2012}}
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| The Indian [[Buddhist]]s, such as [[Dignāga]] in the 5th century and [[Dharmakirti]] in the 7th 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.{{Citation needed|date=January 2012}}
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| ===Descartes===
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| [[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|Bacon]], [[Grosseteste]], and [[Kepler]].<ref name="Theoriesof">''Theories of light, from Descartes to Newton'' A. I. Sabra CUP Archive,1981 pg 48 ISBN 0-521-28436-8, 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.
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| 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' theory of light is regarded as the start of modern physical optics.<ref name="Theoriesof" />
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| ===Particle theory===
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| {{Main|Corpuscular theory of light}}
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| [[Image:PierreGassendi.jpg|thumb|200 px|[[Pierre Gassendi]].]]
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| [[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' 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]].
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| 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 18th century. The particle theory of light led [[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]].
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| ===Wave theory===
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| To explain the origin of colors, [[Robert Hooke]] (1635-1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 [[Micrographia]] ("Observation XI"). 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 ether]]''. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.<ref>Fokko Jan Dijksterhuis, [http://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>
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| [[File:Young Diffraction.png|right|thumb|200px|[[Thomas Young (scientist)|Thomas Young]]'s sketch of the two-slit experiment showing the [[diffraction]] of light. Young's experiments supported the theory that light consists of waves.]]
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| 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]]), and that light could be [[polarized light|polarised]], as if it were a [[transverse wave]]. Young showed by means of a [[double-slit experiment|diffraction experiment]] that light behaved as waves. He also proposed that different [[color|colours]] were caused by different [[wavelength]]s of light, and explained colour vision in terms of three-coloured receptors in the eye.
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| 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.
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| Later, [[Augustin-Jean 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 favour of the wave theory, helping to overturn Newton's corpuscular theory. By the year 1821, Fresnel was able to show via mathematical methods that polarisation could be explained only by the wave theory of light and only if light was entirely transverse, with no longitudinal vibration whatsoever.
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| 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]].
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| 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 | author = David Cassidy, Gerald Holton, James Rutherford | publisher = Birkhäuser | year = 2002 | isbn = 0-387-98756-8 | url = http://books.google.com/?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 20th century.
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| ===Electromagnetic theory as explanation for all types of visible light and all EM radiation===
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| {{Main|Electromagnetic radiation}}
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| [[File:light-wave.svg|360px|thumb|A [[Polarization (waves)|linearly polarised]] light wave frozen in time and showing the two oscillating components of light; an [[electric field]] and a [[magnetic field]] perpendicular to each other and to the direction of motion (a [[transverse wave]]).]]
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| In 1845, [[Michael Faraday]] discovered that the plane of polarisation of linearly polarised 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|year=2003|page=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.
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| 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 behaviour 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 interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.
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| 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).
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| ===Quantum theory===
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| 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 "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 liqht quanta particles [[photon]]s.
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| Eventually the modern theory of [[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, modern physics sees light as something that can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles), and sometimes another macroscopic metaphor (water waves), but is actually something that cannot be fully imagined. 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.
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| ==See also==
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| {{cmn|3|
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| * [[Automotive lighting]]
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| * [[Ballistic photon]]
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| * [[Color temperature]]
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| * [[Electromagnetic spectrum]]
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| * [[Fermat's principle]]
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| * [[Huygens' principle]]
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| * [[International Commission on Illumination]]
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| * ''[[Journal of Luminescence]]''
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| * [[Light beam]] – in particular about light beams visible from the side
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| * [[Light Fantastic (TV series)]]
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| * [[Light mill]]
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| * [[Light pollution]]
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| * [[Light therapy]]
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| * [[Lighting]]
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| * ''[[Luminescence: The Journal of Biological and Chemical Luminescence]]''
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| * [[Photic sneeze reflex]]
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| * [[Photometry (optics)|Photometry]]
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| * [[Photon]]
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| * [[Rights of Light]]
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| * [[Risks and benefits of sun exposure]]
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| * [[Spectroscopy]]
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| * [[Visible spectrum]]
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| * [[Wave–particle duality]]
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| }}
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| ==Notes==
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| {{Reflist|group=nb}}
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| ==References==
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| {{Reflist|2}}
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| ==External links==
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| *{{Commons-inline|Light}}
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| *{{Wiktionary-inline}}
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| *{{Wikiquote-inline}}
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| {{Color topics}}
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| {{Use dmy dates|date=September 2010}}
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| [[Category:Light| ]]
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| {{Link GA|es}}
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| {{Link GA|bg}}
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