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
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.
* 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
* 8 See also * 9 Notes * 10 References * 11 External links
ELECTROMAGNETIC SPECTRUM AND VISIBLE LIGHT
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.
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 .
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 (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
Another, more accurate, measurement of the speed of light was
performed in Europe by
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
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.
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.
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.
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
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:
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
TABLE 1. SI RADIOMETRY UNITS
* v * t * e
QUANTITY UNIT DIMENSION NOTES
NAME SYMBOL NAME SYMBOL SYMBOL
Radiant energy _Q_e joule J M⋅L2⋅T−2 Energy of electromagnetic radiation.
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.
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".
Spectral flux density _E_e,ν
_E_e,λ watt per square metre per hertz
watt per square metre, per metre W⋅m−2⋅Hz−1
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 )
TABLE 2. SI PHOTOMETRY QUANTITIES
* v * t * e
QUANTITY UNIT DIMENSION NOTES
NAME SYMBOL NAME SYMBOL SYMBOL
Luminous energy _Q_v lumen second lm ⋅s T⋅J Units are sometimes called _talbots_.
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
Although the motion of the
HISTORICAL THEORIES ABOUT LIGHT, IN CHRONOLOGICAL ORDER
CLASSICAL GREECE AND HELLENISM
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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
In about 300 BC,
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.
In ancient India , the
The Indian Buddhists , such as
Dignāga in the 5th century and
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.
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
Opticks _ of 1704. His reputation helped the particle theory of
light to hold sway during the 18th century. The particle theory of
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
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
To explain the origin of colors,
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
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
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
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 ).
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,
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.
* ^ 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
* v * t * e
* Spectral colors