The Info List - Light

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LIGHT is electromagnetic radiation within a certain portion of the electromagnetic spectrum . The word usually refers to VISIBLE LIGHT, which is visible to the human eye and is responsible for the sense of sight . Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). This wavelength means a frequency range of roughly 430–750 terahertz (THz).

The main source of light on Earth
is the Sun
. Sunlight
provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems , electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence . For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.

The primary properties of visible light are intensity , propagation direction, frequency or wavelength spectrum , and polarization , while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in a vacuum.

In physics , the term _light_ sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays , X-rays , microwaves and radio waves are also light. Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons and 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.


* 1 Electromagnetic spectrum and visible light * 2 Speed of light

* 3 Optics

* 3.1 Refraction

* 4 Light
sources * 5 Units and measures * 6 Light

* 7 Historical theories about light, in chronological order

* 7.1 Classical Greece and Hellenism * 7.2 Classical India * 7.3 Descartes * 7.4 Particle theory * 7.5 Wave
theory * 7.6 Electromagnetic theory * 7.7 Quantum theory

* 8 See also * 9 Notes * 10 References * 11 External links


Main article: Electromagnetic spectrum Electromagnetic spectrum with light highlighted

Generally, EM radiation, or EMR (the designation "radiation" excludes static electric and magnetic and near fields ), is classified by wavelength into radio , microwave , infrared , the VISIBLE REGION that we perceive as light, ultraviolet , X-rays and gamma rays .

The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

EMR in the visible light region consists of quanta (called photons ) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina, which change triggers the sensation of vision.

There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared
sensing in snakes depends on a kind of natural thermal imaging , in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it.

Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the human eye cannot detect the very short (below 360 nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses (such as insects and shrimp) are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light.

Various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm; children and young adults may perceive ultraviolet wavelengths down to about 310 to 313 nm.

Plant growth is also affected by the color spectrum of light, a process known as photomorphogenesis .


Main article: Speed of light

The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.

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
Ole Rømer
, a Danish physicist, in 1676. Using a telescope , Rømer observed the motions of Jupiter
and one of its moons , Io . Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth
's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau
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 carried out 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 Mount Wilson to Mount San Antonio in California
. The precise measurements yielded a speed of 299,796,000 m/s.

The effective velocity of light in various transparent substances containing ordinary matter , is less than in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists were said to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium , one team at Harvard University and the Rowland Institute for Science in Cambridge, Massachusetts, and the other at the Harvard–Smithsonian Center for Astrophysics , also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.


Main article: Optics

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


Main article: Refraction
An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air. A cloud illuminated by sunlight

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 1 = n 2 sin 2 . {displaystyle 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 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.


Further information: List of light sources
List of light sources

There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight , the radiation emitted by the chromosphere of the Sun
at around 6,000 kelvins (5,730 degrees Celsius; 10,340 degrees Fahrenheit) peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 44% of sunlight energy that reaches the ground is visible. Another example is incandescent light bulbs , which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames , but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.

The peak of the blackbody spectrum is in the deep infrared, at about 10 micrometre wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colours can be seen when metal is heated to "red hot" or "white hot". Blue-white thermal emission is not often seen, except in stars (the commonly seen pure-blue colour in a 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, and is not seen in stars or pure thermal radiation).

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 colorful artificial lighting

Certain other mechanisms can produce light:

* Bioluminescence
* Cherenkov radiation * Electroluminescence * Scintillation * Sonoluminescence * Triboluminescence

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

* Particle–antiparticle annihilation * Radioactive decay


Main articles: Photometry (optics) and Radiometry

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.


* v * t * e



Radiant energy _Q_e joule J M⋅L2⋅T−2 Energy of electromagnetic radiation.

Radiant energy density _w_e joule per cubic metre J/m3 M⋅L−1⋅T−2 Radiant energy per unit volume.

Radiant flux Φe watt W _or_ J/s M⋅L2⋅T−3 Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".

Spectral flux Φe,ν or Φe,λ watt per hertz _or_ watt per metre W/Hz _or_ W/m M⋅L2⋅T−2 _or_ M⋅L⋅T−3 Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1.

Radiant intensity _I_e,Ω watt per steradian W/sr M⋅L2⋅T−3 Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a _directional_ quantity.

Spectral intensity _I_e,Ω,ν or _I_e,Ω,λ watt per steradian per hertz _or_ watt per steradian per metre W⋅sr−1⋅Hz−1 _or_ W⋅sr−1⋅m−1 M⋅L2⋅T−2 _or_ M⋅L⋅T−3 Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a _directional_ quantity.

Radiance _L_e,Ω watt per steradian per square metre W⋅sr−1⋅m−2 M⋅T−3 Radiant flux emitted, reflected, transmitted or received by a _surface_, per unit solid angle per unit projected area. This is a _directional_ quantity. This is sometimes also confusingly called "intensity".

Spectral radiance _L_e,Ω,ν or _L_e,Ω,λ watt per steradian per square metre per hertz _or_ watt per steradian per square metre, per metre W⋅sr−1⋅m−2⋅Hz−1 _or_ W⋅sr−1⋅m−3 M⋅T−2 _or_ M⋅L−1⋅T−3 Radiance of a _surface_ per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a _directional_ quantity. This is sometimes also confusingly called "spectral intensity".

Flux density _E_e watt per square metre W/m2 M⋅T−3 Radiant flux _received_ by a _surface_ per unit area. This is sometimes also confusingly called "intensity".

Spectral irradiance Spectral flux density _E_e,ν or _E_e,λ watt per square metre per hertz _or_ watt per square metre, per metre W⋅m−2⋅Hz−1 _or_ W/m3 M⋅T−2 _or_ M⋅L−1⋅T−3 Irradiance
of a _surface_ per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include Jansky = 10−26 W⋅m−2⋅Hz−1 and solar flux unit (1SFU = 10−22 W⋅m−2⋅Hz−1=104Jy).

Radiosity _J_e watt per square metre W/m2 M⋅T−3 Radiant flux _leaving_ (emitted, reflected and transmitted by) a _surface_ per unit area. This is sometimes also confusingly called "intensity".

Spectral radiosity _J_e,ν or _J_e,λ watt per square metre per hertz _or_ watt per square metre, per metre W⋅m−2⋅Hz−1 _or_ W/m3 M⋅T−2 _or_ M⋅L−1⋅T−3 Radiosity of a _surface_ per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity".

Radiant exitance _M_e watt per square metre W/m2 M⋅T−3 Radiant flux _emitted_ by a _surface_ per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".

Spectral exitance _M_e,ν or _M_e,λ watt per square metre per hertz _or_ watt per square metre, per metre W⋅m−2⋅Hz−1 _or_ W/m3 M⋅T−2 _or_ M⋅L−1⋅T−3 Radiant exitance of a _surface_ per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".

Radiant exposure _H_e joule per square metre J/m2 M⋅T−2 Radiant energy received by a _surface_ per unit area, or equivalently irradiance of a _surface_ integrated over time of irradiation. This is sometimes also called "radiant fluence".

Spectral exposure _H_e,ν or _H_e,λ joule per square metre per hertz _or_ joule per square metre, per metre J⋅m−2⋅Hz−1 _or_ J/m3 M⋅T−1 _or_ M⋅L−1⋅T−2 Radiant exposure of a _surface_ per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence".

Hemispherical emissivity _ε_

1 Radiant exitance of a _surface_, divided by that of a _black body_ at the same temperature as that surface.

Spectral hemispherical emissivity _ε_ν or _ε_λ

1 Spectral exitance of a _surface_, divided by that of a _black body_ at the same temperature as that surface.

Directional emissivity _ε_Ω

1 Radiance _emitted_ by a _surface_, divided by that emitted by a _black body_ at the same temperature as that surface.

Spectral directional emissivity _ε_Ω,ν or _ε_Ω,λ

1 Spectral radiance _emitted_ by a _surface_, divided by that of a _black body_ at the same temperature as that surface.

Hemispherical absorptance _A_

1 Radiant flux _absorbed_ by a _surface_, divided by that received by that surface. This should not be confused with "absorbance ".

Spectral hemispherical absorptance _A_ν or _A_λ

1 Spectral flux _absorbed_ by a _surface_, divided by that received by that surface. This should not be confused with "spectral absorbance ".

Directional absorptance _A_Ω

1 Radiance _absorbed_ by a _surface_, divided by the radiance incident onto that surface. This should not be confused with "absorbance ".

Spectral directional absorptance _A_Ω,ν or _A_Ω,λ

1 Spectral radiance _absorbed_ by a _surface_, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance ".

Hemispherical reflectance _R_

1 Radiant flux _reflected_ by a _surface_, divided by that received by that surface.

Spectral hemispherical reflectance _R_ν or _R_λ

1 Spectral flux _reflected_ by a _surface_, divided by that received by that surface.

Directional reflectance _R_Ω

1 Radiance _reflected_ by a _surface_, divided by that received by that surface.

Spectral directional reflectance _R_Ω,ν or _R_Ω,λ

1 Spectral radiance _reflected_ by a _surface_, divided by that received by that surface.

Hemispherical transmittance _T_

1 Radiant flux _transmitted_ by a _surface_, divided by that received by that surface.

Spectral hemispherical transmittance _T_ν or _T_λ

1 Spectral flux _transmitted_ by a _surface_, divided by that received by that surface.

Directional transmittance _T_Ω

1 Radiance _transmitted_ by a _surface_, divided by that received by that surface.

Spectral directional transmittance _T_Ω,ν or _T_Ω,λ

1 Spectral radiance _transmitted_ by a _surface_, divided by that received by that surface.

Hemispherical attenuation coefficient _μ_ reciprocal metre m−1 L−1 Radiant flux _absorbed_ and _scattered_ by a _volume_ per unit length, divided by that received by that volume.

Spectral hemispherical attenuation coefficient _μ_ν or _μ_λ reciprocal metre m−1 L−1 Spectral radiant flux _absorbed_ and _scattered_ by a _volume_ per unit length, divided by that received by that volume.

Directional attenuation coefficient _μ_Ω reciprocal metre m−1 L−1 Radiance _absorbed_ and _scattered_ by a _volume_ per unit length, divided by that received by that volume.

Spectral directional attenuation coefficient _μ_Ω,ν or _μ_Ω,λ reciprocal metre m−1 L−1 Spectral radiance _absorbed_ and _scattered_ by a _volume_ per unit length, divided by that received by that volume.

See also: SI · Radiometry · Photometry · (Compare )


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Luminous energy _Q_v lumen second lm ⋅s T⋅J Units are sometimes called _talbots_.

Luminous flux
Luminous flux
/ luminous power _Φ_v lumen (= cd⋅sr ) lm J Luminous energy per unit time.

Luminous intensity _I_v candela (= lm/sr) cd J Luminous flux
Luminous flux
per unit solid angle .

Luminance _L_v candela per square metre cd/m2 L−2⋅J Luminous flux
Luminous flux
per unit solid angle per unit _projected_ source area. Units are sometimes called _nits _.

Illuminance _E_v lux (= lm/m2) lx L−2⋅J Luminous flux
Luminous flux
_incident_ on a surface.

Luminous exitance / luminous emittance _M_v lux lx L−2⋅J Luminous flux
Luminous flux
_emitted_ from a surface.

Luminous exposure _H_v lux second lx⋅s L−2⋅T⋅J

Luminous energy density _ω_v lumen second per cubic metre lm⋅s⋅m−3 L−3⋅T⋅J

Luminous efficacy _η_ lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux or power consumption, depending on context.

Luminous efficiency / luminous coefficient _V_


See also: SI · Photometry · Radiometry · (Compare )

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/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.


Main article: Radiation pressure

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. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers. However, in nanometre -scale applications such as nanoelectromechanical systems (NEMS), the effect of light pressure is more significant, and exploiting light pressure to drive NEMS mechanisms and to flip nanometre-scale physical switches in integrated circuits is an active area of research. At larger scales, light pressure can cause asteroids to spin faster, acting on their irregular shapes as on the vanes of a windmill . The possibility of making solar sails that would accelerate spaceships in space is also under investigation.

Although the motion of the Crookes radiometer
Crookes radiometer
was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum. This should not be confused with the Nichols radiometer , in which the (slight) motion caused by torque (though not enough for full rotation against friction) _is_ directly caused by light pressure. As a consequence of light pressure, Einstein in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter. He wrote, “radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief.”



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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 that "The light 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." (from _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_.


In ancient India , the Hindu
schools of Samkhya and Vaisheshika , from around the early centuries AD 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 (_prthivi_), water (_pani_), fire (_agni_), and air (_vayu_) Light
rays are taken to be a stream of high velocity of _tejas_ (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the _tejas_ atoms. 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.


René Descartes
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
. 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. 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.


Main article: Corpuscular theory of light Pierre Gassendi .

Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the _plenum_. He stated in his _Hypothesis of Light_ of 1675 that light was composed of 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 later, after a wave theory of light became firmly established as the model for light (as has been explained, neither a particle or wave theory is fully correct). A translation of Newton's essay on light appears in _The large scale structure of space-time,_ by Stephen Hawking
Stephen Hawking
and George F. R. Ellis .

The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory. Étienne-Louis Malus in 1810 created a mathematical particle theory of polarization. Jean-Baptiste Biot
Jean-Baptiste Biot
in 1812 showed that this theory explained all known phenomena of light polarization. At that time the polarization was considered as the proof of the particle theory.


To explain the origin of colors, Robert Hooke
Robert Hooke
(1635-1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work _ Micrographia
_ ("Observation IX"). In 1672 Hooke suggested that light's vibrations could be perpendicular to the direction of propagation. Christiaan Huygens (1629-1695) worked out a mathematical wave theory of light in 1678, and published it in his _ Treatise on light _ in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the _ Luminiferous ether
Luminiferous ether
_. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. Christiaan Huygens . Thomas Young 's sketch of a double-slit experiment showing diffraction . 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 ). 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
Leonhard Euler
. He argued in _Nova theoria lucis et colorum_ (1746) that diffraction could more easily be explained by a wave theory. In 1816 André-Marie Ampère gave Augustin-Jean Fresnel an idea that the polarization of light can be explained by the wave theory if light were a transverse wave .

Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Siméon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. By the year 1821, Fresnel was able to show via mathematical methods that polarisation could be explained 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. The existence of the hypothetical substance _luminiferous aether_ proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the Michelson–Morley experiment
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. His result supported the wave theory, and the classical particle theory was finally abandoned, only to partly re-emerge in the 20th century.


Main article: Electromagnetic radiation A 3–dimensional rendering of linearly polarised light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion (a transverse wave ).

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 . 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. 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
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.

In the quantum theory, photons are seen as wave packets of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as spectral lines ).


In 1900 Max Planck , attempting to explain black body radiation suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "quanta" (from a Latin word for "how much"). In 1905, Albert Einstein
Albert Einstein
used the idea of light quanta to explain the photoelectric effect , and suggested that these light quanta had a "real" existence. In 1923 Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called Compton scattering ) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926 Gilbert N. Lewis named these light quanta particles photons .

Eventually the modern theory of quantum mechanics came to picture light as (in some sense) _both_ a particle and a wave, and (in another sense), as a phenomenon which is _neither_ a particle nor a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, modern physics sees light as something that can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles), and sometimes another macroscopic metaphor (water waves), but is actually something that cannot be fully imagined. As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both.


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* ^ Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities. * ^ _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 exitance. * ^ _A_ _B_ _C_ _D_ _E_ _F_ _G_ Spectral quantities given per unit frequency are denoted with suffix "ν " (Greek)—not to be confused with suffix "v" (for "visual") indicating a photometric quantity. * ^ _A_ _B_ _C_ _D_ _E_ _F_ _G_ Spectral quantities given per unit wavelength are denoted with suffix "λ " (Greek). * ^ _A_ _B_ Directional quantities are denoted with suffix "Ω " (Greek). * ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: _USA Standard Letter Symbols for Illuminating Engineering_ USAS Z7.1-1967, Y10.18-1967 * ^ _A_ _B_ _C_ Alternative symbols sometimes seen: _W_ for luminous energy, _P_ or _F_ for luminous flux, and _ρ_ or _K_ for luminous efficacy. * ^ _A_ _B_ _C_ "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.


* ^ 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.” * ^ Pal, G. K.; Pal, Pravati (2001). "chapter 52". _Textbook of Practical Physiology_ (1st ed.). Chennai: Orient Blackswan. p. 387. ISBN 978-81-250-2021-9 . Retrieved 11 October 2013. The human eye has the ability to respond to all the wavelengths of light from 400–700 nm. This is called the visible part of the spectrum. * ^ Buser, Pierre A.; Imbert, Michel (1992). _Vision_. MIT Press. p. 50. ISBN 978-0-262-02336-8 . Retrieved 11 October 2013. Light
is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å. * ^ Gregory Hallock Smith (2006). _Camera lenses: from box camera to digital_. SPIE Press. p. 4. ISBN 978-0-8194-6093-6 . * ^ Narinder Kumar (2008). _Comprehensive Physics
XII_. Laxmi Publications. p. 1416. ISBN 978-81-7008-592-8 . * ^ Laufer, Gabriel (13 July 1996). _Introduction to Optics
and Lasers in Engineering_. Cambridge University Press. p. 11. ISBN 978-0-521-45233-5 . Retrieved 20 October 2013. * ^ Bradt, Hale (2004). _Astronomy Methods: A Physical Approach to Astronomical Observations_. Cambridge University Press. p. 26. ISBN 978-0-521-53551-9 . Retrieved 20 October 2013. * ^ Ohannesian, Lena; Streeter, Anthony (9 November 2001). _Handbook of Pharmaceutical Analysis_. CRC Press. p. 187. ISBN 978-0-8247-4194-5 . Retrieved 20 October 2013. * ^ Ahluwalia, V. K.; Goyal, Madhuri (1 January 2000). _A Textbook of Organic Chemistry_. Narosa. p. 110. ISBN 978-81-7319-159-6 . Retrieved 20 October 2013. * ^ Sliney, David H.; Wangemann, Robert T.; Franks, James K.; Wolbarsht, Myron L. (1976). "Visual sensitivity of the eye to infrared laser radiation". _ Journal of the Optical Society of America _. 66 (4): 339–341. doi :10.1364/JOSA.66.000339 . (Subscription required (help)). The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1064 nm. A continuous 1064 nm laser source appeared red, but a 1060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina. * ^ Lynch, David K.; Livingston, William Charles (2001). _ Color
and Light
in Nature_ (2nd ed.). Cambridge, UK: Cambridge University Press. p. 231. ISBN 978-0-521-77504-5 . Retrieved 12 October 2013. Limits of the eye's overall range of sensitivity extends from about 310 to 1050 nanometers * ^ Dash, Madhab Chandra; Dash, Satya Prakash (2009). _Fundamentals Of Ecology 3E_. Tata McGraw-Hill Education. p. 213. ISBN 978-1-259-08109-5 . Retrieved 18 October 2013. Normally the human eye responds to light rays from 390 to 760 nm. This can be extended to a range of 310 to 1,050 nm under artificial conditions. * ^ Saidman, Jean (15 May 1933). "Sur la visibilité de l\'ultraviolet jusqu\'à la longueur d\'onde 3130" . _Comptes rendus de l\'Académie des sciences _ (in French). 196: 1537–9. * ^ "Scientific Method, Statistical Method and the Speed of Light". _Statistical Science_. 15 (3): 254–278. 2000. MR 1847825 . doi :10.1214/ss/1009212817 . * ^ Michelson,, A. A. (January 1927). "Measurements of the velocity of light between Mount Wilson and Mount San Antonio". _Astrophysical Journal_. 65: 1. Bibcode :1927ApJ....65....1M. doi :10.1086/143021 . Retrieved 12 March 2014. * ^ Harvard News Office (2001-01-24). "Harvard Gazette: Researchers now able to stop, restart light". News.harvard.edu. Archived from the original on 28 October 2011. Retrieved 2011-11-08. * ^ http://thulescientific.com/LYNCH%20&%20Soffer%20OPN%201999.pdf * ^ "Reference Solar Spectral Irradiance: Air Mass 1.5". Retrieved 2009-11-12. * ^ Tang, Hong (1 October 2009). "May The Force of Light
Be With You". _IEEE Spectrum_. 46 (10): 46–51. doi :10.1109/MSPEC.2009.5268000 . * ^ See, for example, nano-opto-mechanical systems research at Yale University. * ^ Kathy A. (2004-02-05). "Asteroids Get Spun By the Sun". _Discover Magazine_. * ^ "Solar Sails Could Send Spacecraft \'Sailing\' Through Space". _ NASA
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through the ages: Ancient Greece to Maxwell". * ^ Ptolemy
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