Light
FromAtoms emit and absorb light at characteristic energies. This produces "" in the spectrum of each atom. can be , as in , lamps (such as and , , etc.), and flames (light from the hot gas itself—so, for example, in a gas flame emits characteristic yellow light). Emission can also be , as in a or a microwave .
Deceleration of a free charged particle, such as an , can produce visible radiation: , , and radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible . Certain chemicals produce visible radiation by . In living things, this process is called . For example, 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 . Some substances emit light slowly after excitation by more energetic radiation. This is known as . Phosphorescent materials can also be excited by bombarding them with subatomic particles. is one example. This mechanism is used in and .
illuminated by colorful artificial .
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– annihilation
Units and measures
Light is measured with two main alternative sets of units: consists of measurements of light power at all wavelengths, while measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify intended for human use. The SI units for both systems are summarized in the following tables.
Table 1. SI radiometry units
Quantity
Unit
Dimension
Notes
Name
Symbol[nb 1]
Name
Symbol
Symbol
Qe[nb 2]
J
M⋅L2⋅T−2
Energy of electromagnetic radiation.
we
joule per cubic metre
J/m3
M⋅L−1⋅T−2
Radiant energy per unit volume.
Φe[nb 2]
W = J/s
M⋅L2⋅T−3
Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".
Φe,ν[nb 3]
watt per
W/
M⋅L2⋅T−2
Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.
Φe,λ[nb 4]
watt per metre
W/m
M⋅L⋅T−3
Ie,Ω[nb 5]
watt per
W/
M⋅L2⋅T−3
Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
Ie,Ω,ν[nb 3]
watt per steradian per hertz
W⋅sr−1⋅Hz−1
M⋅L2⋅T−2
Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity.
Ie,Ω,λ[nb 4]
watt per steradian per metre
W⋅sr−1⋅m−1
M⋅L⋅T−3
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 a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
Le,Ω,ν[nb 3]
watt per steradian per square metre per hertz
W⋅sr−1⋅m−2⋅Hz−1
M⋅T−2
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".
Le,Ω,λ[nb 4]
watt per steradian per square metre, per metre
W⋅sr−1⋅m−3
M⋅L−1⋅T−3
Ee[nb 2]
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".
Ee,ν[nb 3]
watt per square metre per hertz
W⋅m−2⋅Hz−1
M⋅T−2
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 (1 Jy = 10−26 W⋅m−2⋅Hz−1) and (1 sfu = 10−22 W⋅m−2⋅Hz−1 = 104 Jy).
Ee,λ[nb 4]
watt per square metre, per metre
W/m3
M⋅L−1⋅T−3
Je[nb 2]
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".
Je,ν[nb 3]
watt per square metre per hertz
W⋅m−2⋅Hz−1
M⋅T−2
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".
Je,λ[nb 4]
watt per square metre, per metre
W/m3
M⋅L−1⋅T−3
Me[nb 2]
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".
Me,ν[nb 3]
watt per square metre per hertz
W⋅m−2⋅Hz−1
M⋅T−2
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".
Me,λ[nb 4]
watt per square metre, per metre
W/m3
M⋅L−1⋅T−3
He
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".
He,ν[nb 3]
joule per square metre per hertz
J⋅m−2⋅Hz−1
M⋅T−1
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".
He,λ[nb 4]
joule per square metre, per metre
J/m3
M⋅L−1⋅T−2
ε
N/A
1
Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.
εν
or
ελ
N/A
1
Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.
εΩ
N/A
1
Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.
εΩ,ν
or
εΩ,λ
N/A
1
Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.
A
N/A
1
Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "".
Aν
or
Aλ
N/A
1
Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "".
AΩ
N/A
1
Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "".
AΩ,ν
or
AΩ,λ
N/A
1
Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "".
R
N/A
1
Radiant flux reflected by a surface, divided by that received by that surface.
Rν
or
Rλ
N/A
1
Spectral flux reflected by a surface, divided by that received by that surface.
RΩ
N/A
1
Radiance reflected by a surface, divided by that received by that surface.
RΩ,ν
or
RΩ,λ
N/A
1
Spectral radiance reflected by a surface, divided by that received by that surface.
T
N/A
1
Radiant flux transmitted by a surface, divided by that received by that surface.
Tν
or
Tλ
N/A
1
Spectral flux transmitted by a surface, divided by that received by that surface.
TΩ
N/A
1
Radiance transmitted by a surface, divided by that received by that surface.
TΩ,ν
or
TΩ,λ
N/A
1
Spectral radiance transmitted by a surface, divided by that received by that surface.
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.
μν
or
μλ
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.
μΩ
reciprocal metre
m−1
L−1
Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
μΩ,ν
or
μΩ,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: · · · ()
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 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 by a quantity called , 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 sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and (CCD) tend to respond to some , or both.
Light pressure
Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by exerts a force of about 3.3 on the object being illuminated; thus, one could lift a with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.-scale applications such as (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. to spin faster,. The possibility of making that would accelerate spaceships in space is also under investigation.
Although the motion of the was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum., in which the (slight) motion caused by torque (though not enough for full rotation against friction) is directly caused by light pressure.[26]
As a consequence of light pressure, [27] 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."
Usually light momentum is aligned with its direction of motion. However, for example in momentum is transverse to direction of propagation.[28]
Historical theories about light, in chronological order
Classical Greece and Hellenism
In the fifth century BC, postulated that everything was composed of ; fire, air, earth and water. He believed that 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.[29]
In about 300 BC, 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. If the beam from the eye travels infinitely fast this is not a problem.[30]
In 55 BC, , a Roman who carried on the ideas of earlier Greek , wrote that "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." (from On the nature of the Universe). Despite being similar to later particle theories, Lucretius's views were not generally accepted. (c. 2nd century) wrote about the of light in his book Optics.[31]
Classical India
In , the schools of and , 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 of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.[32]
On the other hand, the Vaisheshika school gives an of the physical world on the non-atomic ground of , space and time. (See .) 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 refers to sunlight as "the seven rays of the sun".[32]
The Indian , such as in the 5th century and 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.[32]
Descartes
(1596–1650) held that light was a property of the luminous body, rejecting the "forms" of and as well as the "species" of , , and . 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.[33]
Particle theory
(1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. 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 (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 of light (which had been observed by ) by allowing that a light particle could create a localised wave in the .
Newton's theory could be used to predict the of light, but could only explain by incorrectly assuming that light accelerated upon entering a denser because the pull was greater. Newton published the final version of his theory in his of 1704. His reputation helped the to hold sway during the 18th century. The particle theory of light led 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 . 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 and .
The fact that light could be was for the first time qualitatively explained by Newton using the particle theory. in 1810 created a mathematical particle theory of polarization. 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 , (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work ("Observation IX"). In 1672 Hooke suggested that light's vibrations could be to the direction of propagation. (1629–1695) worked out a mathematical wave theory of light in 1678, and published it in his in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the . As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.[34]
The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by ). Young showed by means of a that light behaved as waves. He also proposed that different were caused by different of light, and explained color vision in terms of three-colored receptors in the eye. Another supporter of the wave theory was . He argued in Nova theoria lucis et colorum (1746) that could more easily be explained by a wave theory. In 1816 gave an idea that the polarization of light can be explained by the wave theory if light were a .[35]
Later, Fresnel independently worked out his own wave theory of light, and presented it to the in 1817. added to Fresnel's mathematical work to produce a convincing argument in favor of the wave theory, helping to overturn Newton's corpuscular theory.[ ] By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained by the wave theory of light if 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 could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was , in 1850.[36] 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
In 1845, discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the direction in the presence of a transparent , an effect now known as .. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.
Faraday's work inspired 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 , which contained a full mathematical description of the behavior of electric and magnetic fields, still known as . Soon after, 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 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 ).
Quantum theory
In 1900 , attempting to explain , 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 "" (from a Latin word for "how much"). In 1905, Albert Einstein used the idea of light quanta to explain the , and suggested that these light quanta had a "real" existence. In 1923 showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called ) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926 named these light quanta particles .[39]
Eventually the modern theory of 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.
In February 2018, scientists reported, for the first time, the discovery of a new form of light, which may involve , that could be useful in the development of .
See also
- recommend that radiometric should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or quantities.
- ^ Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.
- ^ Spectral quantities given per unit are denoted with suffix "ν" (Greek)—not to be confused with suffix "v" (for "visual") indicating a photometric quantity.
- ^ Spectral quantities given per unit are denoted with suffix "λ" (Greek).
- ^ Directional quantities are denoted with suffix "Ω" (Greek).
- recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
- ; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the litre, tesla and joule.
- ^ Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.
- (1987). 27 February 2010 at the . Number 17.4. CIE, 4th edition. 978-3-900734-07-7.
By the International Lighting Vocabulary, the definition of light is: "Any radiation capable of causing a visual sensation directly." - . Textbook of Practical Physiology (1st ed.). Chennai: Orient Blackswan. p. 387. 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.
- . MIT Press. p. 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.
- . The Natural Laws of the Universe: Understanding Fundamental Constants. pp. 43–4. :. :. 978-0-387-73454-5.
- . SPIE Press. p. 4. 978-0-8194-6093-6.
- . Laxmi Publications. p. 1416. 978-81-7008-592-8.
- . Introduction to Optics and Lasers in Engineering. p. 11. :. 978-0-521-45233-5. Retrieved 20 October 2013.
- . Cambridge University Press. p. 26. 978-0-521-53551-9. Retrieved 20 October 2013.
- . CRC Press. p. 187. 978-0-8247-4194-5. Retrieved 20 October 2013.
- . Narosa. p. 110. 978-81-7319-159-6. Retrieved 20 October 2013.
- . 66 (4): 339–341. :. :. 1262982. 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.
- (2nd ed.). Cambridge: Cambridge University Press. p. 231. 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
- . Tata McGraw-Hill Education. p. 213. 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.
- [The visibility of the ultraviolet to the wave length of 3130]. (in French). 196: 1537–9.
- . Statistical Science. 15 (3): 254–278. :. 1847825.
- :. :.
- . News.harvard.edu. Archived from on 28 October 2011. Retrieved 8 November 2011.
- (PDF). Thulescientific.com. Retrieved 29 August 2017.
- . Retrieved 12 November 2009.
- :.
- .
- . Discover Magazine.
- . NASA. 31 August 2004.
- . NASA. 9 August 2004.
- ^ P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).
- . The Astrophysical Journal. 17 (5): 315–351. :. :.
- ^ Einstein, A. (1909). On the development of our views concerning the nature and constitution of radiation. Translated in: The Collected Papers of Albert Einstein, vol. 2 (Princeton University Press, Princeton, 1989). Princeton, New Jersey: Princeton University Press. p. 391.
- :. :. 1745-2473.
- 9788183564366.
- .
- 978-0-87169-862-9.
- ^ (PDF). Sifuae.com. Retrieved 29 August 2017.
- ^ Theories of light, from Descartes to Newton A.I. Sabra CUP Archive,1981 p. 48
- , Kluwer Academic Publishers, 2004, 1-4020-2697-8
- ^ James R. Hofmann, André-Marie Ampère: Enlightenment and Electrodynamics, Cambridge University Press, 1996, p. 222.
- . Birkhäuser. 978-0-387-98756-9.
- ^ Longair, Malcolm (2003). . p. 87.
- ^ Cassidy, D (2002). Understanding Physics. Springer Verlag New York.
- Barrow, Gordon M. (1962). (Scanned PDF). McGraw-Hill. 62-12478.
- . Newsweek. Retrieved 17 February 2018.
- . . 359 (6377): 783–786. :. :. :. 29449489.