Light is electromagnetic radiation of a wavelength that is visible to the human eye (in a range from about 380 or 400 nanometres to about 760 or 780 nm).1 In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.23
Five primary properties of light are intensity, frequency or wavelength, polarization, phase and orbital angular momentum.
Light, which exists in tiny "packets" called photons, exhibits properties of both waves and 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.
Contents 1 Speed of light 2 Electromagnetic spectrum 3 Refraction 4 Optics 5 Light sources 6 Units and measures 7 Light pressure 8 Historical theories about light, in chronological order 8.1 Hindu and Buddhist theories 8.2 Greek and Hellenistic theories 8.3 Physical theories 8.4 Particle theory 8.5 Wave theory 8.6 Electromagnetic theory 8.7 The special theory of relativity 8.8 Particle theory revisited 8.9 Quantum theory 8.10 Wave–particle duality 8.11 Quantum electrodynamics 9 Spirituality 10 See also 11 References // Speed of light Main article: Speed of lightThe speed of light in a vacuum is presently defined to be exactly 299,792,458 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.
Different physicists have attempted to measure the speed of light throughout history. 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, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.4 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 Hippolyte Fizeau 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.
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. 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 Bose-Einstein Condensate 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.5
Electromagnetic spectrum Main article: Electromagnetic spectrum Electromagnetic spectrum with light highlightedGenerally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.
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: RefractionRefraction is the bending of light rays when passing from one transparent material to another. It is described by Snell's Law:
where θ1 is the angle between the ray and the normal in the first medium, θ2 is the angle between the ray and the normal in the second medium, and n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not 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.
Light refraction is the main basis of measurement for gloss. Gloss is measured using a glossmeter.
Optics Main article: OpticsThe 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.
Light sources See also: List of light sources A cloud illuminated by sunlightThere 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. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units [1] and roughly 40% of sunlight is visible), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. 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 heated to "red hot" or "white hot". Blue thermal emission is not often seen. The commonly seen blue colour in a gas flame or a welder's 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 "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, 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, as in a laser or a microwave maser.
Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.
Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.
Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.
A city illuminated by light bulbsCertain other mechanisms can produce light:
scintillation electroluminescence sonoluminescence triboluminescence Cherenkov radiationWhen the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
Radioactive decay Particle–antiparticle annihilation Units and measures Main articles: Photometry (optics) and RadiometryLight is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarized in the following tables.
SI radiometry units Quantity Symbol SI unit Abbr. Notes Radiant energy Q joule J energy Radiant flux Φ watt W radiant energy per unit time, also called radiant power Radiant intensity I watt per steradian W·sr−1 power per unit solid angle Radiance L watt per steradian per square metre W·sr−1·m−2 power per unit solid angle per unit projected source area.called intensity in some other fields of study.
Irradiance E, I watt per square metre W·m−2 power incident on a surface.sometimes confusingly called "intensity".
Radiant exitance / Radiant emittance M watt per square metre W·m−2 power emitted from a surface. Radiosity J or Jλ watt per square metre W·m−2 emitted plus reflected power leaving a surface Spectral radiance Lλ or Lν watt per steradian per metre3 orwatt per steradian per square metre per hertz
W·sr−1·m−3 orW·sr−1·m−2·Hz−1
commonly measured in W·sr−1·m−2·nm−1 Spectral irradiance Eλ or Eν watt per metre3 or watt per square metre per hertz W·m−3 or W·m−2·Hz−1 commonly measured in W·m−2·nm−1 SI photometry units v • d • e Quantity Symbol SI unit Abbr. Notes Luminous energy Qv lumen second lm·s units are sometimes called talbots Luminous flux F lumen (= cd·sr) lm also called luminous power Luminous intensity Iv candela (= lm/sr) cd an SI base unit Luminance Lv candela per square metre cd/m2 units are sometimes called "nits" Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface Luminous efficacy lumen per watt lm/W ratio of luminous flux to radiant flux See also SI · Photometry · RadiometryThe 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 cells 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/m2) 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 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 devices (CCD) tend to respond to some infrared, ultraviolet or both.
Light pressure Main article: Radiation pressureLight exerts physical pressure on objects in its path, a phenomenon which can be 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 c, the speed of light. Due to the magnitude of c, the effect of light pressure is negligible for everyday objects. For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U. S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers6. However, in nanometer-scale applications such as NEMS, the effect of light pressure is more pronounced, and exploiting light pressure to drive NEMS mechanisms and to flip nanometer-scale physical switches in integrated circuits is an active area of research7.
At larger scales, light pressure can cause asteroids to spin faster8, 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 investigation910.
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 vacuum11. This should not be confused with the Nichols radiometer, in which the motion is directly caused by light pressure12.
Historical theories about light, in chronological order Hindu and Buddhist theoriesIn ancient India, the Hindu schools of Samkhya and Vaisheshika, from around the 6th–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 atomicity 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 atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) 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 Vishnu Purana refers to sunlight as "the seven rays of the sun".
The Indian Buddhists, 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, similar to the modern concept of photons, though they also viewed all matter as being composed of these light/energy particles.
It is written in the Rigveda that light consists of three primary colors. "Mixing the three colours, ye have produced all the objects of sight!"13
Greek and Hellenistic theoriesIn the fifth century BC, Empedocles postulated that everything was composed of 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.
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.
In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, 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.
Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics.14 Aristotle"s theory of light would eclipse all others.citation needed
Physical theoriesRené Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Witelo as well as the "species" of Bacon, Grosseteste, and Kepler.15 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 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.16
Particle theory Main article: Corpuscular theory of lightPierre 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 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 Grimaldi) by allowing that a light particle could create a localised wave in the aether.
Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the 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 when the wave theory of light was firmly established. A translation of his essay appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.
Wave theoryIn the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens 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 Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.
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.The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarized, if it were a transverse wave. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye.
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.
Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon 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 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 luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.
Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850.17 His result supported the wave theory, and the classical particle theory was finally abandoned.
Electromagnetic theory A linearly polarized 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).In 1845, Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation.18 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.19 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 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.
The special theory of relativityThe 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 Galileo, which stated that all speeds were relative to the speed of the observer. In 1905, Albert Einstein 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 special theory of relativity, which advanced humankind's understanding of space and time. Einstein also demonstrated a previously unknown fundamental equivalence between energy and mass with his famous equation
where E is energy, m is, depending on the context, the rest mass or the relativistic mass, and c is the speed of light in a vacuum.
Particle theory revisitedAnother experimental anomaly was the photoelectric effect, by which light striking a metal surface ejected electrons from the surface, causing an electric current to flow across an applied voltage. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the frequency, rather than the intensity, 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 wave–particle duality and much of quantum mechanics.
Quantum theoryA 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 black bodies. Physicists struggled with this problem, which later became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max Planck developed a new theory of black-body radiation that explained the observed spectrum. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by
where h is Planck's constant, λ is the wavelength and c is the speed of light. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:
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 Nobel Prize in Physics for his part in the founding of quantum theory.
Wave–particle dualityThe modern theory that explains the nature of light includes the notion of wave–particle duality, described by Albert Einstein in the early 1900s, based on his study of the photoelectric effect and Planck's results. Einstein asserted that the energy of a photon is proportional to its frequency. 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 Louis de Broglie in 1924 that the scientific community realized that electrons 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 electrodynamicsThe quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920s and 1930s, and culminated with the development during the 1940s of the theory of quantum electrodynamics, or QED. This so-called quantum field theory is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena. QED was developed primarily by physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Shin-Ichiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.
Spirituality Further information: Light and darkness An intricate display for the feast of St. Thomas at Kallara Pazhayapalli in Kottayam, Kerala, India dramatically illustrates the importance of light in religion.The sensory perception of light plays a central role in spirituality (vision, enlightenment, darshan, Tabor Light). The presence of light as opposed to its absence (darkness) is a common metaphor of good and evil, knowledge and ignorance, and similar concepts. God's thought system is understood to be light and a higher vibrational frequency. Therefore light is understanding which can be achieved through knowledge, right-mindedness and forgiveness. Whereas darkness is ignorance which is a form of judgement (wrong-mindedness) and as such is resistance to light. These ideas are prevalent in both Eastern and Western spirituality.
See also Wikimedia Commons has media related to: Light Look up light in Wiktionary, the free dictionary. Wikiquote has a collection of quotations related to: Light Automotive lighting Ballistic photon Color temperature Electromagnetic spectrum Fermat's principle Huygens' principle International Commission on Illumination Journal of Luminescence Light beam – in particular about light beams visible from the side Light Fantastic (TV series) Light pollution Light therapy Lighting Luminescence: The Journal of Biological and Chemical Luminescence Photic sneeze reflex Photometry Rights of Light Risks and benefits of sun exposure Spectrometry Spectroscopy Visible light Wave–particle duality References ^ CIE (1987). International Lighting Vocabulary. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7. By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.” ^ Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 9780819460936, http://books.google.com/?id=6mb0C0cFCEYC&pg=PA4 ^ Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN 9788170085928, http://books.google.com/?id=IryMtwHHngIC&pg=PA1416#v=onepage&q= ^ Scientific Method, Statistical Method and the Speed of Light. Statistical Science 2000, Vol. 15, No. 3, 254–278 ^ http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html ^ Tang, Hong X. (October 2009), "May the Force of Light Be with You", IEEE Spectrum: pp. 41 – 45, http://www.spectrum.ieee.org/semiconductors/devices/photonics-breakthrough-for-silicon-chips, retrieved 2010-09-07 . ^ See, for example, nano-opto-mechanical systems research at Yale University. ^ Kathy A. (2004-02-05). "Asteroids Get Spun By the Sun". Discover Magazine. http://discovermagazine.com/2004/feb/asteroids-get-spun-by-the-sun/. ^ "Solar Sails Could Send Spacecraft 'Sailing' Through Space". NASA. 2004-08-31. http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html. ^ "NASA team successfully deploys two solar sail systems". NASA. 2004-08-09. http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html. ^ 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. ^ Vyasa, Krishna-Dwai (2008-03), The Mahabharata of Krishna-Dwaipayana Vyasa First Book Adi Parva, The Echo Library, p. 41, ISBN 978-1-40687-045-9, http://books.google.com/?id=NYg_CBpCCHAC , Section III , p. 41 ^ Ptolemy and A. Mark Smith (1996), Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary, Diane Publishing, p. 23, ISBN 0-871-69862-5 ^ Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0521284368, 9780521284363 ^ 'Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0521284368, 9780521284363 ^ David Cassidy, Gerald Holton, James Rutherford (2002), Understanding Physics, Birkhäuser, ISBN 0387987568, http://books.google.com/?id=rpQo7f9F1xUC&pg=PA382 ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87. ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87The ring of bright spots in the centre of the image is known as the String of Pearls' - a glowing ring 6 trillion miles in diameter encircling the leftovers of a supernova.
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WINCHESTER, Va., Sept. 8 (UPI) -- The light bulb dreamed up by Thomas Alva Edison is on its last legs as a product made in the United States, a victim of regulations, General Electric said. United States - Thomas Edison - General Electric - Incandescent light bulb - Thomas Alva Edison
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Light definition, something that makes things visible or affords illumination: See more.
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WINCHESTER, VA. - The last major GE factory making ordinary incandescent light bulbs in the United States is closing this month, marking a small, sad exit for a product and company that can trace their roots to Thomas Alva Edison's innovations in the 1870s. Light - Business - Business and Economy - China - Incandescent light bulb
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By Robinson Newspapers Staff Ghost Light Theatricals has chosen "Ch-Ch-Changes" as the theme for its 2010-2011 season, its eighth as a company but its first full season in The Ballard Underground at 2220 N.W. Market St. While settling into their new home, adding a show to their season and shifting from nomads into Ballard's only theater company, the upcoming season represents a transformed Ghost ...
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light n. Physics . Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be
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Rockford, Ill.- The Rockford IceHogs weekly talk show, Bud Light Hog Talk, is returning to Swilligan's Pub in downtown Rockford. The 2010-11 edition will feature 18 shows from 6-7 p.m. on Mondays beginning on Monday, Oct. 11.
Taming Light 8 I m trying experiments now to isolate particular refraction patterns against a dark background For new viewers These are light refraction patterns or caustics formed by a beam of light passing through a shaped and textured plastic form Various coloured dyes were added to the clear plastic as it hardened adding further distortions to the shapes The light pattern is captured directly on to 35mm film by removing the camera lens and putting the transparent object in its place Please note these are not computer generated images but a true analogue of the way light is refracted by the objects I create
VICKY LEANDROS - MONTAGMORGEN
Light and Lens: Photography in the Digital Age Focal Press
Light Reading (www.lightreading.com), owned by Techweb, is the ultimate source for technology and financial analysis of the communications industry leading the media ...
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Point two laser beams so that they cross each other, and each goes through as if the other one did not exist. Light rays cannot interact with other light rays--or can they? With the help of a single atom, physicists have devised a system in which one light beam can turn another on or off. Such a light switch could serve as the basic component of futuristic optical quantum computers and may help ...
let your little light shine
Shitty camera fun
Mother Teresa: Come Be My Light - The Private Writings of the Saint of Calcutta Doubleday Religion
The way we think about light has changed through the course of history, from the theories of the ancient Greeks to the theories of Albert Einstein. ...
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KULAIJAYA, Sept 7 (Bernama) -- The engine of a light aircraft which crashed at an oil palm estate near Kampung Sri Gunung Pulai Tuesday, leaving a woman trainee pilot dead and a flight examiner injured, had stalled in mid-air, according to an eyewitness.
when lights runing away other <a href= http www flickr com photos hawee ta3kees sets 72157600699871961 >Creative Effect < a> <b><a href= http www flickr com people hawee ta3kees >Copyright Statement< a>< b>
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Sun, Wind & Light: Architectural Design Strategies, 2nd Edition Wiley
A tiny optical device built into a silicon chip has achieved the slowest light propagation on a chip to date, reducing the speed of light by a factor of 1,200 in a study reported in Nature Photonics (published online September 5 and in the November print issue). By Tim Stephens
<a href= http bighugelabs com flickr scout php username=24304582 N00 amp combined=1 >Explored < a> Thank you <a href= http www flickr com photos sakshisharma >sakshi sharma< a> <a href= http www flickr com photos potogeek >Nagaraj B R< a> and <a href= http www flickr com photos sravi in 3218213794 >sravi in< a> for tagging me From my flickr name many ask if I am from Light and Life Academy When I coined the name I was not aware of such an organization Thanks to flickr and friends who helped improving the quality of photos I take Friends I had a tremendous learning from all your photos Helped to see the world around in more beautiful way Completed my Engineering in 1994 at CIT Coimbatore Now I am a software architect at Sasken Bangalore Most of the time I live in the past school days nostalgic We are group of 6 fellows go fast with our cycle in a row in our village We go to school tuition swimming pool wide well and a dam after 15 km At times we had walked 10km From Dindigul to Chinnalapatti Did mountaineering claimed Sirumallai <a href= http www flickr com photos murali art 3039976896 >View of Sirumalai from my home < a> Lot of activities Also we were the toppers in our school So no issue from parents I did well in the first year of the college LIFE started in my second year Had FULL FUN Lot of experiments Resulted in arrears Put all efforts to clear them Then learned how to balance essence of life with studies ha full of fun Now it is helping to spend time in flickr balancing home and office work Rajini he is my all time favorite I remember going to Rajini movies during my college semester exams It was double the thrill Before leaving to the movie I and my friend Nirmal used to go to some worried studious fellows and ask if they we studied something in the last chapter and tell them that we have studied and it is very important Next day we go to the exam hall with full of thiruneer
Luka Skywalker
Hands of Light: A Guide to Healing Through the Human Energy Field Bantam
Photo taken on Sept. 7, 2010 shows the debris of a light aeroplance near an oil palm plantation in Kulai in southern Johore State in southern part of Peninsula Malaysia.
I ve started a new series called quot <b>I got the Sun< b> quot A story through the images uniting poetry and photography As you see first <b>I got a tiny light < b> The story and more pictures are coming soon No digital effects and processing Thank you all for your beautiful impressions and all the feedback on the first line I appreciate them much Welcome to the next line of the story
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