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	==Theories about light==
	===Indian theories===

	In ], the philosophical schools of ] and ], from around the ]–5th century BC, 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 ] of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.

	On the other hand, the Vaisheshika school gives an ] of the physical world on the non-atomic ground of ], space and time. (See '']''.) The basic ]s are those of earth (''prthivı''), water (''apas''), fire (''tejas''), and air (''vayu''), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. 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. Around the first century BC, the '']'' correctly refers to ] as the "the seven rays of the sun".

	Later in 499, ], who proposed a ] ] of ] in his '']'', wrote that the planets and the ] do not have their own light but reflect the light of the ].

	The Indian ]s, such as ] in the 5th century and ] in the 7th century, developed a type of ] 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, similar to the modern concept of ]s, though they also viewed all matter as being composed of these light/energy particles.

	===Greek and Hellenistic theories===
	{{main\|Emission theory (vision)}}

	In the fifth century BC, ] postulated that everything was composed of ]; fire, air, earth and water. He believed that ] 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.

	In about 300 BC, ] 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.

	In 55 BC, ], a Roman who carried on the ideas of earlier Greek ], wrote:

	"''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''

	Despite being similar to later particle theories, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.

	] (c. 2nd century) wrote about the ] of light in his book ''Optics'', and developed a theory of vision whereby objects are seen by rays of light emanating from the eyes.<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 = 0-871-69862-5}}</ref>

	===Optical theory===
	{{main\|Book of Optics\|Physics in medieval Islam\|History of optics}}

	The ], ] (965-1040), known as ''Alhacen'' or ''Alhazen'' in the West, in his '']'' (1021), developed a broad theory that explained ], using ] and ], which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He described the ] and invented the ], which produces an inverted image, and used it as an example to support his argument.<ref></ref> This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes. Alhacen held light rays to be streams of minute ]<ref name=Rashed>{{Citation \|last=Rashed \|first=Roshdi \|year=2007 \|title=The Celestial Kinematics of Ibn al-Haytham \|journal=Arabic Sciences and Philosophy \|volume=17 \|pages=7–55 \|publisher=] \|doi=10.1017/S0957423907000355 }}: {{quote\|"In his optics ‘‘the smallest parts of light’’, as he calls them, retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."}}</ref> that travelled at a ].<ref name=MacTutor>{{MacTutor\|id=Al-Haytham\|title=Abu Ali al-Hasan ibn al-Haytham}}</ref><ref name=MacKay>{{citation\|title=Scientific Method, Statistical Method and the Speed of Light\|first1=R. J.\|last1=MacKay\|first2=R. W.\|last2=Oldford\|journal=Statistical Science\|volume=15\|issue=3\|date=August 2000\|pages=254–78}}</ref><ref name=Hamarneh>Sami Hamarneh (March 1972). Review of Hakim Mohammed Said, ''Ibn al-Haitham'', '']'' '''63''' (1), p. 119.</ref> He improved ]'s theory of the refraction of light, and went on to discover the laws of refraction.

	He also carried out the first experiments on the dispersion of light into its constituent colors. His major work ''Kitab al-Manazir'' (''Book of Optics'') was translated into ] in the ], as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain ], and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon, known as the ]. Because of his extensive experimental research on optics, Ibn al-Haytham is considered the "father of modern ]".<ref>R. L. Verma (1969). ''Al-Hazen: father of modern optics''.</ref>

	Ibn al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny energy particles<ref name=Rashed/> travelling in straight lines, are reflected from objects into our eyes.<ref name=MacTutor/> He understood that light must travel at a large but finite velocity,<ref name=MacTutor/><ref name=MacKay/><ref name=Hamarneh/> and that refraction is caused by the velocity being different in different substances.<ref name=MacTutor/> He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.

	] (980-1037) agreed that the speed of light is finite, as he "observed that if the perception of light is due to the emission of some sort of particles by a luminous source, the speed of light must be finite."<ref name=Sarton>], ''Introduction to the History of Science'', Vol. 1, p. 710.</ref> ] (973-1048) also agreed that light has a finite speed, and he was the first to discover that the speed of light is much faster than the ].<ref name=Biruni>{{MacTutor\|id=Al-Biruni\|title=Al-Biruni}}</ref> In the late 13th and early 14th centuries, ] (1236-1311) and his student ] (1260-1320) continued the work of Ibn al-Haytham, and they were the first to give the correct explanations for the ] phenomenon.<ref name=Biruni />

	===The 'plenum'===
	] (1596-1650) held that light was a disturbance of the ''plenum'', the continuous substance of which the universe was composed. In 1637 he published a theory of the ] 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 ] waves. 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. As a result, Descartes' theory is often regarded as the forerunner of the wave theory of light.

	===Particle theory===
	] (Alhazen, 965-1040) proposed a particle theory of light in his '']'' (1021). He held light rays to be streams of minute ]<ref name=Rashed/> that travel in straight lines at a ].<ref name=MacTutor/><ref name=MacKay/><ref name=Hamarneh/> He states in his optics that "the smallest parts of light," as he calls them, "retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."<ref name=Rashed/> ] (980-1037) also proposed that "the perception of light is due to the emission of some sort of particles by a luminous source".<ref name=Sarton/>

	] (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. ] 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 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 ] of light (which had been observed by ]) by allowing that a light particle could create a localised wave in the ].

	Newton's theory could be used to predict the ] of light, but could only explain ] by incorrectly assuming that light accelerated upon entering a denser ] because the ] pull was greater. Newton published the final version of his theory in his '']'' of 1704. His reputation helped the ] to hold sway during the 18th century. The particle theory of light led ] 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 when the wave theory of light was firmly established. A translation of his essay appears in ''The large scale structure of space-time,'' by ] and ].

	===Wave theory===

	In the 1660s, ] published a ] theory of light. ] worked out his own 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 '']''. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

	] of light. Young's experiments supported the theory that light consists of waves.]]

	The wave theory predicted that light waves could interfere with each other like ] waves (as noted around 1800 by ]), and that light could be ]. Young showed by means of a ] that light behaved as waves. He also proposed that different ]s were caused by different ]s of light, and explained color vision in terms of three-colored receptors in the eye.

	Another supporter of the wave theory was ]. He argued in ''Nova theoria lucis et colorum'' (1746) that ] could more easily be explained by a wave theory.

	Later, ] independently worked out his own wave theory of light, and presented it to the ] in 1817. ] added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.

	The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the ] was proposed, but its existence was cast into strong doubt in the late nineteenth century by the ].

	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 ] could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was ], in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

	===Electromagnetic theory===

	] light wave frozen in time and showing the two oscillating components of light; an ] and a ] perpendicular to each other and to the direction of motion (a ]).]]

	In 1845, ] discovered that the angle of polarization of a beam of light as it passed through a polarizing material could be altered by a ] field, an effect now known as ]. This was the first evidence that light was related to ]. 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.

	Faraday's work inspired ] 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 '']'', which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as ]. Soon after, ] confirmed Maxwell's theory experimentally by generating and detecting ] 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.

	===The special theory of relativity===

	The wave theory was wildly successful in explaining nearly all optical and electromagnetic phenomena, and was a great triumph of nineteenth century physics. By the late nineteenth century, however, a handful of experimental anomalies remained that could not be explained by or were in direct conflict with the wave theory. One of these anomalies involved a controversy over the speed of light. The constant speed of light predicted by Maxwell's equations and confirmed by the Michelson-Morley experiment contradicted the mechanical laws of motion that had been unchallenged since the time of ], which stated that all speeds were relative to the speed of the observer. In 1905, ] resolved this paradox by revising the Galilean model of space and time <!-- ] ] --> to account for the constancy of the speed of light. Einstein formulated his ideas in his ], which radically altered humankind's understanding of ] and ]. Einstein also demonstrated a previously unknown fundamental ] between ] and ] with his famous equation
	:<math>E = mc^2 \, </math>

	where ''E'' is energy, ''m'' is rest mass, and ''c'' is the ] in a vacuum.

	===Particle theory revisited===

	Another experimental anomaly was the ], by which light striking a metal surface ejected electrons from the surface, causing an ] to flow across an applied ]. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the '']'', rather than the '']'', of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations clearly contradicted the wave theory, and for years physicists tried in vain to find an explanation. In 1905, Einstein solved this puzzle as well, this time by resurrecting the particle theory of light to explain the observed effect. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially by great skepticism among established physicists. But eventually Einstein's explanation of the photoelectric effect would triumph, and it ultimately formed the basis for ] and much of ].

	===Quantum theory===

	A third anomaly that arose in the late 19th century involved a contradiction between the wave theory of light and measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called ]. Physicists struggled with this problem, which later became known as the ], unsuccessfully for many years. In 1900, ] developed a new theory of ] that explained the observed spectrum correctly. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of ]. These packets were called ], and the particle of light was given the name ], to correspond with other particles being described around this time, such as the ] and ]. A
	photon has an energy, ''E'', proportional to its frequency, ''f'', by

	:<math>E = hf = \frac{hc}{\lambda} \,\! </math>

	where ''h'' is ], <math>\lambda</math> is the wavelength and ''c'' is the ]. Likewise, the momentum ''p'' of a photon is also proportional to its frequency and inversely proportional to its wavelength:

	:<math>p = { E \over c } = { hf \over c } = { h \over \lambda }. </math>

	As it originally stood, this theory did not explain the simultaneous wave- and particle-like natures of light, though Planck would later work on theories that did. In 1918, Planck received the ] for his part in the founding of quantum theory.

	===Wave–particle duality===

	The modern theory that explains the nature of light includes the notion of ], described by ] in the early 1900s, based on his study of the ] and Planck's results. Einstein asserted that the energy of a photon is proportional to its ]. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, and it was not until a bold proposition by ] in 1924 that the scientific community realized that ] also exhibited wave–particle duality. The wave nature of electrons was experimentally demonstrated by Davission and Germer in 1927. Einstein received the Nobel Prize in 1921 for his work with the wave–particle duality on photons (especially explaining the photoelectric effect thereby), and de Broglie followed in 1929 for his extension to other particles.

	===Quantum electrodynamics===

	The quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920s and 1930's, and culminated with the development during the 1940s of the theory of ], or QED. This so-called ] is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena.
	QED was developed primarily by physicists ], ], ], and ]. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.

	== Light pressure ==		== Light pressure ==

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For other uses, see Light (disambiguation).

File:PrismAndLight.jpg

A beam of white light (entering upwards from the right) is dispersed into its constituent colors by its passage through a prism. The fainter beam of white light exiting to the upper right has been reflected (without dispersion) off the first surface of the prism.

Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm), or up to 380–750 nm. In the broader field of physics, light is often used to refer to electromagnetic radiation of all wavelengths, whether visible or not.

Three primary properties of light are:

Light can exhibit properties of both waves and particles (photons). This property is referred to as wave–particle duality. The study of light, known as optics, is an important research area in modern physics. hellomoto hdfjhsncorqwufn gzsczngtoeu grd

Refraction

Note, n = 1 in a vacuum and n > 1 in a transparent substance, where n is the index of refraction.

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 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 lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.

Optics

Main article: Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light as well as much enjoyment.

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Light pressure

Main article: Radiation pressure

Light pushes on objects in its way, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause asteroids to spin faster, acting on their irregular shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate spaceships in space is also under investigation.

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. This should not be confused with the Nichols radiometer, in which the motion is directly caused by light pressure.

Spirituality

File:Kallara Pazhyapalli Light.JPG

An intricate display for the feast of St.Thomas at Kallara Pazhayapalli in Kottayam, Kerala dramatically illustrates the importance of light in religion

The sensory perception of light plays a central role in spirituality (vision, enlightenment, darshan, Tabor Light), and the presence of light as opposed to its absence (darkness) is a common Western metaphor of good and evil, knowledge and ignorance, and similar concepts.

References

Cecie Starr (2005). Biology: Concepts and Applications. Thomson Brooks/Cole. ISBN 053446226X.
Kathy A. (02.05.2004). "Asteroids Get Spun By the Sun". Discover Magazine. {{cite web}}: Check date values in: |date= (help)
"Solar Sails Could Send Spacecraft 'Sailing' Through Space". NASA. 08.31.2004. {{cite web}}: Check date values in: |date= (help)
"NASA team successfully deploys two solar sail systems". NASA. 08.9.2004. {{cite web}}: Check date values in: |date= (help)
P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).
Nichols, E.F & Hull, G.F. (1903) The Pressure due to Radiation, The Astrophysical Journal,Vol.17 No.5, p.315-351

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