# Visible Light(ஒளி)...!

Source:  Visible Light(ஒளி)...!    Tag:  visible light frequency range

# Light

The  Sun is Earth's primary source of light
Light or  visible light is  electromagnetic radiation that is  visible to the  human eye, and is responsible for the sense of  sight. [1] Visible light has  wavelength in a range from about 380  nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz. In physics, the term  light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. [2] [3]
Primary properties of light are  intensity, propagation direction,  frequency or  wavelength  spectrum, and  polarisation, while its  speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants of nature.
Light, which is emitted and absorbed 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.

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## Speed of light

The speed of light in a  vacuum is 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, Rømer observed the motions of  Jupiter and one of its  moonsIo. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of  Earth's orbit. [4] Unfortunately, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.
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 team 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

Electromagnetic spectrum with light highlighted
Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and  near fields) is classified by wavelength into  radiomicrowaveinfrared, the  visible region we perceive as light,  ultravioletX-raysand  gamma rays.
The behaviour 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 behaviour depends on the amount of energy per quantum it carries.

## 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.

### Refraction

An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.
Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by Snell's Law:
$n_1\sin\theta_1 = n_2\sin\theta_2\ .$
where  θ1 is the angle between the ray and the  surface normal in the first medium,  θ2 is the angle between the ray and the surface normal in the second medium, and n 1 and n 2 are the  indices of refractionn = 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, spectaclescontact lensesmicroscopes and  refracting telescopes are all examples of this manipulation.

## Light sources

cloud illuminated by  sunlight
There are  many sources of light. The most common light sources are thermal: a body at a given  temperature emits a characteristic spectrum of  black-body radiation. Examples include  sunlight (the radiation emitted by the  chromosphere of the  Sun at around 6,000  Kelvin peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units  [6] 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 colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is  heated to "red hot" or "white hot". Blue  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 diodesgas discharge lamps (such as  neon lamps and  neon signsmercury-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 radiationsynchrotron 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.
city illuminated by  artificial lighting
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:

## Units and measures

Light 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 standardised model of human brightness perception. Photometry is useful, for example, to quantify  Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.
Quantity Symbol[nb 1] SI unit Symbol Dimension Notes
Radiant energy Qe[nb 2] joule J M⋅L2⋅T−2 energy
Spectral power Φ[nb 2][nb 3] watt per metre W⋅m−1 M⋅L⋅T−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 M⋅L2⋅T−3 power per unit solid angle.
Spectral intensity I[nb 3] watt per steradian per metre W⋅sr−1⋅m−1 M⋅L⋅T−3 radiant intensity per wavelength.
Radiance Le watt per steradian persquare metre W⋅sr−1m−2 M⋅T−3 power per unit solid angle per unit projected source area.
confusingly called " intensity" in some other fields of study.
or
L[nb 4]
or
metre per  hertz
W⋅sr−1m−3
or
W⋅sr−1⋅m−2Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and eitherwavelength or frequency.

Irradiance Ee[nb 2] watt per square metre W⋅m−2 M⋅T−3 power incident on a surface, also called radiant flux density.
sometimes confusingly called " intensity" as well.
or
E[nb 4]
watt per metre3
or
watt per square metre per hertz
W⋅m−3
or
W⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
commonly measured in W⋅m−2nm−1
or 10−22W⋅m−2⋅Hz−1, known as solar flux unit.[nb 5]

Me[nb 2] watt per square metre W⋅m−2 M⋅T−3 power emitted from a surface.
M[nb 3]
or
M[nb 4]
watt per metre3
or
watt per square
metre per hertz
W⋅m−3
or
W⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
power emitted from a surface per wavelength or frequency.

Radiosity Je orJ[nb 3] watt per square metre W⋅m−2 M⋅T−3 emitted plus reflected power leaving a surface.
Radiant exposure He joule per square metre J⋅m−2 M⋅T−2
Radiant energy density ωe joule per metre3 J⋅m−3 M⋅L−1⋅T−2
Table 2. SI photometry units
Quantity Symbol[nb 6] SI unit Symbol Dimension Notes
Luminous energy Qv [nb 7] lumen second lm⋅s T⋅J units are sometimes called talbots
Luminous flux Φv [nb 7] lumen (= cd⋅sr) lm J also called luminous power
Luminous intensity Iv candela (= lm/sr) cd [nb 8] an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2⋅J units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx L−2⋅J used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx L−2⋅J used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J
Luminous energy density ωv lumen second per metre3 lm⋅sm−3 L−3⋅T⋅J
Luminous efficacy η [nb 7] lumen per watt lm/W M−1⋅L−2⋅T3⋅J ratio of luminous flux to radiant flux
Luminous efficiency V 1 also called luminous coefficient
The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The  cone 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/m 2) 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  infraredultraviolet or both.

## Light pressure

Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by  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. pennywith laser pointers, but doing so would require about 30 billion 1-mW laser pointers. [7]   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 research. [8]
At larger scales, light pressure can cause  asteroids to spin faster, [9] 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. [10] [11]
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. [12] This should not be confused with the  Nichols radiometer, in which the motion  is directly caused by light pressure. [13]

## Historical theories about light, in chronological order

### Hindu and Buddhist theories

In  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 colours. "Mixing the three colours, ye have produced all the objects of sight!" [14]

### Greek and Hellenistic theories

In 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. [15]

### Physical theories

René 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. [16] 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. [17]

### Particle theory

Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s.  Isaac Newtonstudied 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 theory

In 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  diffractionof 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  polarised, if it were a  transverse wave. Young showed by means of a  diffraction experiment that light behaved as waves. He also proposed that different  colours were caused by different  wavelengths of light, and explained colour vision in terms of three-coloured 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 polarisation 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 lightcould not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was  Léon Foucault, in 1850. [18] His result supported the wave theory, and the classical particle theory was finally abandoned.

### Electromagnetic theory

linearly polarised light wave frozen in time and showing the two oscillating components of light; an  electric field and a  magnetic fieldperpendicular to each other and to the direction of motion (a  transverse wave).
In 1845,  Michael Faraday discovered that the plane of polarisation of linearly polarised light is rotated when the light rays travel along the  magnetic field direction in the presence of a transparent  dielectric, an effect now known as Faraday rotation. [19] 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. [20] 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.

### Special theory of relativity

Albert Einstein had proposed the  Theory of relativity.
The wave theory was 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 proposing that  space and  time appeared to be changeable entities, which accounted for the constancy of the speed of light. Einstein also proposed a previously unknown fundamental  equivalence between  energy and  mass with his famous equation
$E = mc^2 \,$
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 revisited

Another 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 with 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 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  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
$E = hf = \frac{hc}{\lambda} \,\!$
where  h is  Planck's constantλ 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:
$p = { E \over c } = { hf \over c } = { h \over \lambda }.$
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 duality

The 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 realised 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 electrodynamics

The 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 FeynmanFreeman DysonJulian Schwinger, and  Shin-Ichiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.

## Spirituality

An intricate display for the feast of  St. Thomas at Kallara Pazhayapalli in Kottayam, KeralaIndia.
The term light has been used in spirituality ( visionenlightenmentdarshanTabor Light). Bible commentators such as Ritenbaugh see the presence of light as a metaphor of  truthgood and evilknowledge and  ignorance. [21] In the  first Chapter of the Bible, Elohim is described as creating light by  fiat and seeing the light to be good. In Eastern religion,  Diwali — the festival of lights — is a celebration of the victory of light over darkness. [22] A  mantra in  Bṛhadāraṇyaka Upaniṣad (1.3.28) urges God to 'from darkness, lead us unto Light'.

## Notes

1. ^ Standards organizations recommend that radiometric quantities should be denoted with a suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
2. a b c d e Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant emittance.
3. a b c d e f Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek) to indicate a spectral concentration. Spectral functions of wavelength are indicated by "(λ)" in parentheses instead, for example in spectral transmittancereflectance and responsivity.
4. a b c Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek)—not to be confused with the suffix "v" (for "visual") indicating a photometric quantity.
5. ^ NOAA / Space Weather Prediction Center includes a definition of the solar flux unit (SFU).
6. ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
7. a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
8. ^ "J" is the recommended symbol for the dimension of luminous intensity in the International System of Units.

## References

1. ^ 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.”
2. ^ Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 9780819460936
3. ^ Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416,ISBN 9788170085928
4. ^ Scientific Method, Statistical Method and the Speed of Light. Statistical Science 2000, Vol. 15, No. 3, 254–278
5. ^ Harvard News Office (2001-01-24). "Harvard Gazette: Researchers now able to stop, restart light". News.harvard.edu. Retrieved 2011-11-08.
6. ^ http://thulescientific.com/LYNCH%20&%20Soffer%20OPN%201999.pdf
7. ^ Tang, Hong X. (October 2009), "May the Force of Light Be with You"IEEE Spectrum: pp. 41 – 45, retrieved 7 September 2010.
8. ^ See, for example, nano-opto-mechanical systems research at Yale University.
9. ^ Kathy A. (2004-02-05). "Asteroids Get Spun By the Sun"Discover Magazine.
10. ^
11. ^ "NASA team successfully deploys two solar sail systems"NASA. 2004-08-09.
12. ^ P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).
13. ^ Nichols, E.F & Hull, G.F. (1903) The Pressure due to RadiationThe Astrophysical Journal,Vol.17 No.5, p.315–351.
14. ^ Vyasa, Krishna-Dwai (2008-03), The Mahabharata of Krishna-Dwaipayana Vyasa First Book Adi Parva, The Echo Library, p. 41, ISBN 978-1-40687-045-9Section III , p. 41
15. ^ 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
16. ^ Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0-521-28436-8, 9780521284363
17. ^ 'Theories of light, from Descartes to Newton A. I. Sabra CUP Archive,1981 pg 48 ISBN 0-521-28436-8, 9780521284363
18. ^ David Cassidy, Gerald Holton, James Rutherford (2002), Understanding Physics, Birkhäuser, ISBN 0387987568
19. ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87.
20. ^ Longair, Malcolm. Theoretical Concepts in Physics (2003) p. 87
21. ^ "Light as Metaphor of Truth (Forerunner Commentary)". Bible Tools. Retrieved 2011-11-08.
22. ^ "Religions - Hinduism: Diwali". BBC. 2010-10-20. Retrieved 2011-11-08.