"Visible light" redirects here. For light that cannot be seen with human eye, see Electromagnetic radiation. For other uses, see Light (disambiguation) and Visible light (disambiguation).
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.[1] Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared (with longer wavelengths) and the ultraviolet(with shorter wavelengths).[2][3] Often, infrared and ultraviolet are also called light.
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 light box and of power systems, electric lighting has all but replaced firelight. Some species of animals generate their own light, called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.
Primary properties of visible light are intensity, propagation direction, frequency or wavelengthspectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters 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 vacuum.[citation needed]
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[4][5] 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 asoptics, is an important research area in modern physics.
Contents
[hide]- 1 Electromagnetic spectrum and visible light
- 2 Speed of light
- 3 Optics
- 3.1 Refraction
- 4 Light box
- 5 Units and measures
- 6 Light pressure
- 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 as explanation for all types of visible light and all EM radiation
- 7.7 Quantum theory
- 8 See also
- 9 Notes
- 10 References
- 11 External links
Electromagnetic spectrum and visible light
Main article: Electromagnetic spectrum
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 behaviour 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 behaviour 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 lead 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[6][7] to as broadly as 380 to 800 nm.[8][9] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm,[10] children and young adults may perceive ultraviolet wavelengths down to about 310 to 313 nm.[11][12][13]
Speed of light
Main article: 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. 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, 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.[14] 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 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 Mount Wilson to Mount San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.[15]
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, Mass., and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge.[16] 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.
Optics
Main article: Optics
The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and theaurora borealis offer many clues as to the nature of light.
Refraction
Main article: Refraction
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:
where 
is the angle between the ray and the surface normal in the first medium, 
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 avacuum and n > 1 in a transparent substance.
is the angle between the ray and the surface normal in the first medium, 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 avacuum 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 notorthogonal (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 sources
Further information: List of light sources
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. A simple thermal source is 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[17] and roughly 44% of sunlight energy that reaches the ground is visible.[18] 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 10micrometer 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 LED 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 microwavemaser.
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.
Certain other mechanisms can produce light box:
- 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
Units and measures
Main articles: Photometry (optics) and Radiometry
Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometrymeasures 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.
Table 1. SI radiometry quantities
| Quantity | Unit | Dimension | Notes | |||||
|---|---|---|---|---|---|---|---|---|
| Name | Symbol[nb1] | Name | Symbol | Symbol | ||||
| Radiant energy | Qe[nb 2] | joule | J | M⋅L2⋅T−2 | Energy of electromagnetic radiation. | |||
| Radiant energy density | we | joule per cubic metre | J/m3 | M⋅L−1⋅T−2 | Radiant energy per unit volume. | |||
| Radiant flux | Φe[nb 2] | 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[nb 3] or Φe,λ[nb 4] | 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 | Ie,Ω[nb 5] | watt per steradian | W/sr | M⋅L2⋅T−3 | Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is adirectional quantity. | |||
| Spectral intensity | Ie,Ω,ν[nb 3] or Ie,Ω,λ[nb 4] | 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⋅m−2⋅nm−1. This is adirectional quantity. | |||
| Radiance | Le,Ω[nb 5] | watt per steradian per square metre | W⋅sr−1⋅m−2 | M⋅T−3 | Radiant flux emitted, reflected, transmitted or received by asurface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity". | |||
| Spectral radiance | Le,Ω,ν[nb 3] or Le,Ω,λ[nb 4] | 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 adirectional quantity. This is sometimes also confusingly called "spectral intensity". | |||
| Irradiance | Ee[nb 2] | watt per square metre | W/m2 | M⋅T−3 | Radiant flux received by asurface per unit area. This is sometimes also confusingly called "intensity". | |||
| Spectral irradiance | Ee,ν[nb 3] or Ee,λ[nb 4] | 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. The former is commonly measured in 10−22 W⋅m−2⋅Hz−1, known as solar flux unit, and the latter in W⋅m−2⋅nm−1.[nb 6] This is sometimes also confusingly called "spectral intensity". | |||
| Radiosity | Je[nb 2] | watt per square metre | W/m2 | M⋅T−3 | Radiant flux leaving (emitted, reflected and transmitted by) asurface per unit area. This is sometimes also confusingly called "intensity". | |||
| Spectral radiosity | Je,ν[nb 3] or Je,λ[nb 4] | 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 | Me[nb 2] | watt per square metre | W/m2 | M⋅T−3 | Radiant flux emitted by asurface 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 | Me,ν[nb 3] or Me,λ[nb 4] | 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 surfaceper 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 | He | joule per square metre | J/m2 | M⋅T−2 | Radiant energy received by asurface per unit area, or equivalently irradiance of asurface integrated over time of irradiation. This is sometimes also called "radiant fluence". | |||
| Spectral exposure | He,ν[nb 3] or He,λ[nb 4] | 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 surfaceper 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 bodyat the same temperature as that surface. | |||||
| Spectral hemispherical emissivity | εν or ελ | 1 | Spectral exitance of a surface, divided by that of a black bodyat the same temperature as that surface. | |||||
| Directional emissivity | εΩ | 1 | Radiance emitted by a surface, divided by that emitted by ablack body at the same temperature as that surface. | |||||
| Spectral directional emissivity | εΩ,ν or εΩ,λ | 1 | Spectral radiance emitted by asurface, divided by that of ablack body at the same temperature as that surface. | |||||
| Hemispherical absorptance | A | 1 | Radiant flux absorbed by asurface, divided by that received by that surface. This should not be confused with "absorbance". | |||||
| Spectral hemispherical absorptance | Aν or Aλ | 1 | Spectral flux absorbed by asurface, divided by that received by that surface. This should not be confused with "spectral absorbance". | |||||
| Directional absorptance | AΩ | 1 | Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance". | |||||
| Spectral directional absorptance | AΩ,ν or AΩ,λ | 1 | Spectral radiance absorbed by asurface, 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 asurface, divided by that received by that surface. | |||||
| Spectral hemispherical reflectance | Rν or Rλ | 1 | Spectral flux reflected by asurface, divided by that received by that surface. | |||||
| Directional reflectance | RΩ | 1 | Radiance reflected by a surface, divided by that received by that surface. | |||||
| Spectral directional reflectance | RΩ,ν or RΩ,λ | 1 | Spectral radiance reflected by asurface, divided by that received by that surface. | |||||
| Hemispherical transmittance | T | 1 | Radiant flux transmitted by asurface, divided by that received by that surface. | |||||
| Spectral hemispherical transmittance | Tν or Tλ | 1 | Spectral flux transmitted by asurface, divided by that received by that surface. | |||||
| Directional transmittance | TΩ | 1 | Radiance transmitted by asurface, divided by that received by that surface. | |||||
| Spectral directional transmittance | TΩ,ν or TΩ,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 andscattered by a volume per unit length, divided by that received by that volume. | |||
| Spectral hemispherical attenuation coefficient | μν or μλ | reciprocal metre | m−1 | L−1 | Spectral radiant flux absorbedand scattered by a volume per unit length, divided by that received by that volume. | |||
| Directional attenuation coefficient | μΩ | reciprocal metre | m−1 | L−1 | Radiance absorbed and scatteredby a volume per unit length, divided by that received by that volume. | |||
| Spectral directional attenuation coefficient | μΩ,ν or μΩ,λ | reciprocal metre | m−1 | L−1 | Spectral radiance absorbed andscattered by a volume per unit length, divided by that received by that volume. | |||
| See also: SI · Radiometry · Photometry · (Compare) | ||||||||
Table 2. SI photometry quantities
| Quantity | Unit | Dimension | Notes | |||||
|---|---|---|---|---|---|---|---|---|
| Name | Symbol[nb7] | Name | Symbol | Symbol | ||||
| Luminous energy | Qv [nb 8] | lumen second | lm⋅s | T⋅J [nb 9] | Units are sometimes called talbots. | |||
| Luminous flux / Luminous power | Φv [nb 8] | lumen (= cd⋅sr) | lm | J [nb 9] | Luminous energy per unit time. | |||
| Luminous intensity | Iv | candela (= lm/sr) | cd | J [nb 9] | Luminous power per unit solid angle. | |||
| Luminance | Lv | candela per square metre | cd/m2 | L−2⋅J | Luminous power per unit solid angle per unit projectedsource area. Units are sometimes called nits. | |||
| Illuminance | Ev | lux (= lm/m2) | lx | L−2⋅J | Luminous power incident on a surface. | |||
| Luminous exitance / Luminous emittance | Mv | lux | lx | L−2⋅J | Luminous power emitted from a surface. | |||
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | ||||
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s⋅m−3 | L−3⋅T⋅J | ||||
| Luminous efficacy | η [nb 8] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux. | |||
| Luminous efficiency / Luminous coefficient | V | 1 | ||||||
| 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.
Light pressure
Main article: Radiation 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. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.[19] However, in nano metre-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.[20]
At larger scales, light pressure can cause asteroids to spin faster,[21] 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.[22][23]
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.[24] 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.[25]
Historical theories about light, in chronological order
Classical Greece and Hellenism
 | This section needs additional citations for verification. (May 2011) |
In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphroditemade 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.[26]
Classical India
| This section needs additional citations for verification. (May 2011) |
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. (SeeIndian 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 thetejas atoms.[citation needed] The Vishnu Purana refers to sunlight as "the seven rays of the sun".[citation needed]
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.[citation needed]
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.[27] 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.[27]
Particle theory
Main article: Corpuscular theory of light
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 theplenum. 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 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 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.
Wave theory
To explain the origin of colors, Robert Hooke (1635-1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 Micrographia ("Observation XI"). 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. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.[28]
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. He argued in Nova theoria lucis et colorum (1746) thatdiffraction could more easily be explained by a wave theory.
In 1815 Ampere gave Fresnel an idea that the polarization of light can be explained by the wave theory if light were a transverse wave.
Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to theAcadémie 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.
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.[29] His result supported the wave theory, and the classical particle theory was finally abandoned, only to partly re-emerge in the 20th century.
Electromagnetic theory as explanation for all types of visible light and all EM radiation
Main article: Electromagnetic radiation
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.[30] 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.[30] 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.
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).
Quantum theory
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 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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