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| '''Light''' is ] of a ] that is ] to the ] (in a range from about 380 or 400 ]s to about 760 or 780 nm).<ref>] (1987). . 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> In ], 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 = 9780819460936 | page = 4 | url = http://books.google.com/books?id=6mb0C0cFCEYC&pg=PA4 }}</ref><ref>{{cite book | title = Comprehensive Physics XII | author = Narinder Kumar | publisher = Laxmi Publications | year = 2008 | isbn = 9788170085928 | page = 1416 | url = http://books.google.com/books?id=IryMtwHHngIC&pg=PA1416#v=onepage&q=&f=false }}</ref> | |||
| Four primary properties of light are ], ] or ], ], and ] | |||
| Light, which exists in tiny "packets" called ], exhibits properties of both ]s and ]. This property is referred to as the ]. The study of light, known as ], is an important research area in modern physics. | |||
| ==Speed of light== | |||
| {{Main|Speed of light}} | |||
| The speed of light in a ] is presently defined to be exactly 299,792,458 ] (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the ] is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited. | |||
| Different physicists have attempted to measure the speed of light throughout history. ] attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by ], a Danish physicist, in 1676. Using a telescope, Ole observed the motions of ] and one of its ]s, ]. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of ]'s orbit.<ref>''''. Statistical Science 2000, Vol. 15, No. 3, 254–278</ref> Unfortunately, its size was not known at that time. If Ole had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. | |||
| Another, more accurate, measurement of the speed of light was performed in Europe by ] in 1849. 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 313,000,000 m/s. | |||
| ] used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. ] conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating ]s to measure the ] it took light to make a round trip from ] to ] in ]. The precise measurements yielded a speed of 299,796,000 m/s. | |||
| Two independent teams of physicists were able to bring light to a complete standstill by passing it through a ] of the element rubidium, one led by Dr. Lene Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.{{Citation needed|date=January 2010}} | |||
| == Electromagnetic spectrum == | |||
| {{Main|Electromagnetic spectrum}} | |||
| ] with light highlighted]] | |||
| Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and ]) is classified by wavelength into ], ], ], the ] we perceive as light, ], ]s and ]. | |||
| The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. | |||
| ==Refraction== | |||
| ''Main article: ]'' | |||
| Refraction is the bending of light rays when passing from one transparent material to another. It is described by ]: | |||
| :<math>n_1\sin\theta_1 = n_2\sin\theta_2\ .</math> | |||
| where <math>\theta_1</math> is the angle between the ray and the ] in the first medium, <math>\theta_2</math> is the angle between the ray and the ] in the second medium, and n<sub>1</sub> and n<sub>2</sub> are the ], ''n'' = 1 in a ] and ''n'' > 1 in a ] ]. | |||
| 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 ] (or rather ]) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as ]. | |||
| The refractive quality of ]es is frequently used to manipulate light in order to change the apparent size of images. ]es, ], ]es, ]s and ]s are all examples of this manipulation. | |||
| Light refraction is the main basis of measurement for ]. Gloss is measured using a ]. | |||
| ==Optics== | |||
| {{Main|Optics}} | |||
| The study of light and the interaction of light and ] is termed ]. The observation and study of ] such as ]s and the ] offer many clues as to the nature of light as well as much enjoyment. | |||
| ==Light sources== <!-- This section is linked from ] --> | |||
| {{See also|List of light sources}} | |||
| ] illuminated by ]]] | |||
| There are ]. The most common light sources are thermal: a body at a given ] emits a characteristic spectrum of ] radiation. Examples include ] (the radiation emitted by the ] of the ] at around 6,000 ] peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units <sup></sup> and roughly 40% of sunlight is visible), ]s (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in ]. The peak of the blackbody spectrum is in the infrared 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 color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is ]ed to "red hot" or "white hot". Blue ] emission is not often seen. The commonly seen blue colour in a ] flame or a ] torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm). | |||
| Atoms emit and absorb light at characteristic energies. This produces "]s" in the spectrum of each atom. ] can be ], as in ]s, ] lamps (such as ]s and ]s, ]s, etc.), and flames (light from the hot gas itself—so, for example, ] in a gas flame emits characteristic yellow light). Emission can also be ], as in a ] or a microwave ]. | |||
| Deceleration of a free charged particle, such as an ], can produce visible radiation: ], ], and ] radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible ]. | |||
| Certain chemicals produce visible radiation by ]. In living things, this process is called ]. For example, ] 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 ]. Some substances emit light slowly after excitation by more energetic radiation. This is known as ]. | |||
| Phosphorescent materials can also be excited by bombarding them with subatomic particles. ] is one example of this. This mechanism is used in ] ]s and ]s. | |||
| ] illuminated by ]]] | |||
| Certain other mechanisms can produce light: | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include: | |||
| * ] | |||
| * Particle–] annihilation | |||
| == Units and measures == | |||
| {{Main|Photometry (optics)|Radiometry}} | |||
| Light is measured with two main alternative sets of units: ] consists of measurements of light power at all wavelengths, while ] measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify ] intended for human use. The SI units for both systems are summarized in the following tables. | |||
| {{SI radiometry units}} | |||
| {{SI light units}} | |||
| 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 ]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 ] by a quantity called ], 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 ] sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and ]s (CCD) tend to respond to some ], ] or both. | |||
| ==Historical theories about light, in chronological order== | |||
| ===Hindu and Buddhist theories=== | |||
| In ], the ] 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 atoms are those of earth (''prthivı''), water (''pani''), fire (''agni''), 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 '']'' 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. | |||
| It is written in the ] that light consists of three primary colors. "Mixing the three colours, ye have produced all the objects of sight!"<ref>{{citation | |||
| |title=The Mahabharata of Krishna-Dwaipayana Vyasa First Book Adi Parva | |||
| |first1=Krishna-Dwai | |||
| |last1=Vyasa | |||
| |publisher=The Echo Library | |||
| |isbn=978-1-40687-045-9 | |||
| |page=41 | |||
| |url=http://books.google.be/books?id=NYg_CBpCCHAC}}, | |||
| </ref> | |||
| ===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|History of optics}} | |||
| {{See also|Book of Optics|Physics in medieval Islam}} | |||
| ] proved that light travels in straight lines through optical experiments.]] | |||
| The ], ] (965–1040), known as ''Alhacen'' or ''Alhazen'' in the West, developed a broad theory of ] based on ] and ] in his '']'' (1021). Ibn al-Haytham provided the first correct description of how vision works,<ref>Bashar Saad, Hassan Azaizeh, Omar Said (October 2005). "Tradition and Perspectives of Arab Herbal Medicine: A Review", ''Evidence-based Complementary and Alternative Medicine'' '''2''' (4), p. 475-479 . ].</ref> explaining that it is not due to objects being seen by rays of ] from the eyes, as ] and ] had assumed, but due to light rays entering the eyes.<ref>D. C. Lindberg, ''Theories of Vision from al-Kindi to Kepler'', (Chicago, Univ. of Chicago Pr., 1976), pp. 60-7.</ref> Ibn al-Haytham postulated that every point on an illuminated surface radiates light rays in all directions, but that only one ray from each point can be seen: the ray that strikes the eye perpendicularly. The other rays strike at different angles and are not seen. He conducted ]s to support his argument, which included the development of apparatus such as the ] and ], which produces an inverted image.<ref></ref> Alhacen held light rays to be streams of minute particles that "lack all sensible qualities except energy"<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> and travel 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|doi=10.1214/ss/1009212817}}</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 ] of light, and went on to describe the ], though this was earlier discovered by ] (c. 940-1000) several decades before him.<ref>K. B. Wolf, "Geometry and dynamics in refracting systems", ''European Journal of Physics'' '''16''', p. 14-20, 1995.</ref><ref name=rashed90>R. Rashed, "A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses", '']'' '''81''', p. 464–491, 1990.</ref> | |||
| ]'s manuscript showing his discovery of the law of ] (]).]] | |||
| 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 an 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> | |||
| ] developed the ] and ] for his experiments on light.]] | |||
| 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. | |||
| Ibn al-Haytham's optical model of light was "the first comprehensive and systematic alternative to Greek optical theories."<ref>D. C. Lindberg, "Alhazen's Theory of Vision and its Reception in the West", ''Isis'', 58 (1967), p. 322.</ref> He initiated a ] in optics and ],<ref name=Sabra>{{citation|last1=Sabra|first1=A. I.|author1-link=A. I. Sabra|last2=Hogendijk|first2=J. P.|year=2003|title=The Enterprise of Science in Islam: New Perspectives|pages=85–118|publisher=]|isbn=0262194821|oclc=237875424}}</ref><ref name=Hatfield>{{Citation |last=Hatfield |first=Gary |contribution=Was the Scientific Revolution Really a Revolution in Science? |editor1-last=Ragep |editor1-first=F. J. |editor2-last=Ragep |editor2-first=Sally P. |editor3-last=Livesey |editor3-first=Steven John |year=1996 |title=Tradition, Transmission, Transformation: Proceedings of Two Conferences on Pre-modern Science held at the University of Oklahoma |page=500 |publisher=] |isbn=9004091262 |oclc=19740432 234073624 234096934}}</ref><ref>{{Citation|journal=The Medieval History Journal|volume=9|issue=1|pages=89–98|year=2006|doi=10.1177/097194580500900105|title=The Gaze in Ibn al-Haytham|first=Gérard|last=Simon}}</ref><ref>{{citation|title=Burning Instruments: From Diocles to Ibn Sahl|first=Hélèna|last=Bellosta|journal=Arabic Sciences and Philosophy|year=2002|volume=12|pages=285–303|publisher=]|doi=10.1017/S095742390200214X}}</ref><ref>{{citation|title=Portraits of Science: A Polymath in the 10th Century|first=Roshdi|last=Rashed|journal=]|date=2 August 2002|volume=297|issue=5582|page=773|doi=10.1126/science.1074591|pages=773|pmid=12161634}}</ref><ref>{{Citation |last=Lindberg |first=David C. |year=1967 |title=Alhazen's Theory of Vision and Its Reception in the West |journal=] |volume=58 |issue=3 |pages=321–341 |doi=10.1086/350266 }}</ref> also known as the 'Optical Revolution',<ref>{{citation|title=The Dialogue of Civilizations in the Birth of Modern Science|first=Arun|last=Bala|publisher=]}}</ref> and laid the foundations for a ].<ref name=Verma/><ref>{{Citation |last=Toomer |first=G. J. |year=1964 |date=December 1964 |title=Review: ''Ibn al-Haythams Weg zur Physik'' by Matthias Schramm |journal=] |volume=55 |issue=4 |pages=463–465 |doi=10.1086/349914}}</ref> As such, he is often regarded as the "father of modern optics."<ref name=Verma>R. L. Verma "Al-Hazen: father of modern optics", ''Al-Arabi'', 8 (1969): 12-13.</ref> | |||
| ] (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>{{MacTutor|id=Al-Farisi|title=Al-Farisi}}</ref> | |||
| ] (1596–1650) held that light was a ] property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Whitelo as well as the "species" of Bacon, Grosseteste, and Kepler.<ref>''Theories of light, from Descartes to Newton'' A. I. Sabra CUP Archive,1981 pg 48 ISBN 0521284368, 9780521284363</ref> 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.{{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' theory of light is regarded as the start of modern physical optics.<ref>'Theories of light, from Descartes to Newton'' A. I. Sabra CUP Archive,1981 pg 48 ISBN 0521284368, 9780521284363</ref> | |||
| ===Particle theory=== | |||
| {{Main|Corpuscular theory of light}} | |||
| ] (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 ] (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=== <!-- {{main|Wave theory of light}} - currently a redirect back to this page, but it could be resurrected if this section gets too big in future --> | |||
| 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. | |||
| ]'s sketch of the two-slit experiment showing the ] 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 ], if it were a ]. 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. By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained only by the wave theory of light and only if light was entirely transverse, with no longitudinal vibration whatsoever. | |||
| 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.<ref>{{cite book | title = Understanding Physics | author = David Cassidy, Gerald Holton, James Rutherford | publisher = Birkhäuser | year = 2002 | isbn = 0387987568 | url = http://books.google.com/books?id=rpQo7f9F1xUC&pg=PA382 }}</ref> 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 plane of polarization of linearly polarized light is rotated when the light rays travel along the ] direction in the presence of a transparent ], an effect now known as ].<ref>Longair, Malcolm. ''Theoretical Concepts in Physics'' (2003) p. 87.</ref> This was the first evidence that light was related to ]. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.<ref>Longair, Malcolm. ''Theoretical Concepts in Physics'' (2003) p. 87</ref> 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 advanced 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, depending on the context, the ] or the ], 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 appeared to contradict 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 Davisson 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== | |||
| {{Main|Radiation pressure}} | |||
| Light pushes on objects in its path, 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 ]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 | |||
| | author = Kathy A. | work = Discover Magazine | date = 02.05.2004}}</ref> acting on their irregular shapes as on the vanes of a ]. The possibility to make ]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 = ] | 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 = 08.9.2004}}</ref> | |||
| Although the motion of the ] 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 ], in which the motion ''is'' directly caused by light pressure.<ref>Nichols, E.F & Hull, G.F. (1903) , ''The Astrophysical Journal'',Vol.17 No.5, p.315–351</ref> | |||
| ==Spirituality== | |||
| {{See|Light and darkness}} | |||
| ] at Kallara Pazhayapalli in Kottayam, ], ] dramatically illustrates the importance of light in religion.]] | |||
| The sensory perception of light plays a central role in spirituality (], ], ], ]). The presence of light as opposed to its absence (]) is a common metaphor of ], ] and ], and similar concepts. This idea is prevalent in both Eastern and Western spirituality. | |||
| ==See also== | |||
| {{commons|Light}}{{Wiktionary}}{{wikiquote}} | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * '']'' | |||
| * ] – in particular about light beams visible from the side | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * '']'' | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| ==References== | |||
| {{reflist|2}} | |||
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Revision as of 01:29, 19 April 2010
ALLY BIRD IS HOT!!!!!!!!!!!!!!