Inphysics, the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not.[4][5] In this sense,gamma rays,X-rays,microwaves andradio waves are also light. The primary properties of light areintensity, propagation direction, frequency or wavelengthspectrum, andpolarization. Itsspeed in vacuum,299792458 m/s, is one of the fundamentalconstants of nature.[6] All electromagnetic radiation exhibits some properties of bothparticles and waves. Single, massless elementary particles, orquanta, of light calledphotons can be detected with specialized equipment; phenomena like interference are described by waves. Most everyday interactions with light can be understood usinggeometrical optics;quantum optics, is an important research area inmodern physics.
The main source of natural light on Earth is theSun. Historically, another important source of light for humans has beenfire, from ancient campfires to modernkerosene lamps. With the development ofelectric lights andpower systems, electric lighting has effectively replaced firelight.
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 ofquanta (calledphotons) 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 moleculeretinal 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 naturalthermal 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 360nm and the internal lens below 400 nm. Furthermore, therods andcones located in theretina 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 420–680 nm[7][8] to as broadly as 380–800 nm.[9][10] Under ideal laboratory conditions, people can see infrared up to at least 1,050 nm;[11] children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm.[12][13][14]
Plant growth is also affected by the colour spectrum of light, a process known asphotomorphogenesis.
Beam of sun light inside the cavity of Rocca ill'Abissu atFondachelli-Fantina, Sicily
The speed of light invacuum is defined to be exactly299792458 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.
Differentphysicists 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 byOle Rømer, a Danish physicist, in 1676. Using atelescope, Rømer observed the motions ofJupiter and one of itsmoons,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.[15] 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 of227000000 m/s.
Another more accurate measurement of the speed of light was performed in Europe byHippolyte Fizeau in 1849.[16] Fizeau directed a beam of light at a mirror several kilometers away. A rotatingcog 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 as313000000 m/s.
Léon Foucault carried out an experiment which used rotating mirrors to obtain a value of298000000 m/s[16] 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 fromMount Wilson toMount San Antonio in California. The precise measurements yielded a speed of299796000 m/s.[17]
The effective velocity of light in various transparent substances containing ordinarymatter, 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 aBose–Einstein condensate of the elementrubidium, one team atHarvard University and theRowland Institute for Science in Cambridge, Massachusetts and the other at theHarvard–Smithsonian Center for Astrophysics, also in Cambridge.[18] 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 andmatter is termedoptics. Optics has different forms appropriate to different circumstances.Geometrical optics, appropriate for understanding things like eyes, lenses, cameras,fiber optics, and mirrors, works well when the wavelength of light is small in comparison to the objects it interacts with.Physical optics incorporates wave properties and is needed understand diffraction and interference.Quantum optics applies when studying individual photons interacting with matter.[19]: 33
Surface scattering
Atransparent object allows light totransmit or pass through. Conversely, anopaque object does not allow light to transmit through and insteadreflecting orabsorbing the light it receives. Most objects do not reflect or transmit lightspecularly and to some degreescatters the incoming light, which is calledglossiness. Surface scattering is caused by thesurface roughness of the reflecting surfaces, and internal scattering is caused by the difference ofrefractive index between the particles andmedium inside the object. Like transparent objects,translucent objects allow light to transmit through, but translucent objects also scatter certain wavelength of light via internal scattering.[20]
Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.
Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described bySnell's Law:
where θ1 is the angle between the ray and the surfacenormal in the first medium, θ2 is the angle between the ray and the surface normal in the second medium and n1 and n2 are theindices of refraction,n = 1 in avacuum andn > 1 in atransparentsubstance.
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 asrefraction.
There are many sources of light. A body at a given temperature emits a characteristic spectrum ofblack-body radiation. A simple thermal source issunlight, the radiation emitted by thechromosphere of theSun at around 6,000 K (5,730 °C; 10,340 °F). Solar radiation peaks in the visible region of theelectromagnetic spectrum when plotted in wavelength units,[21] and roughly 44% of the radiation that reaches the ground is visible.[22] Another example isincandescent 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 inflames, but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.
The peak of the black-body spectrum is in the deep infrared, at about 10micrometre 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-whitethermal emission is not often seen, except in stars (the commonly seen pure-blue colour in agas flame or awelder'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).
Deceleration of a free charged particle, such as anelectron, can produce visible radiation:cyclotron radiation,synchrotron radiation andbremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visibleCherenkov radiation. Certain chemicals produce visible radiation bychemoluminescence. In living things, this process is calledbioluminescence. 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 asfluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known asphosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles.Cathodoluminescence is one example. This mechanism is used incathode-ray tubetelevision sets andcomputer monitors.
Light is measured with two main alternative sets of units:radiometry consists of measurements of light power at all wavelengths, whilephotometry measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantifyIllumination (lighting) intended for human use.
The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. Thecone 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 rawpower by a quantity calledluminous 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 aphotocell sensor does not necessarily correspond to what is perceived by the human eye and without filters which may be costly, photocells andcharge-coupled devices (CCD) tend to respond to someinfrared,ultraviolet or both.
Light exerts physical pressure on objects in its path, a phenomenon which can be deduced byMaxwell'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 byc, the speed of light. Due to the magnitude ofc, the effect of light pressure is negligible for everyday objects. For example, a one-milliwattlaser pointer exerts a force of about 3.3piconewtons on the object being illuminated; thus, one could lift aU.S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.[23] However, innanometre-scale applications such asnanoelectromechanical 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.[24] At larger scales, light pressure can causeasteroids to spin faster,[25] acting on their irregular shapes as on the vanes of awindmill. The possibility of makingsolar sails that would accelerate spaceships in space is also under investigation.[26][27]
Although the motion of theCrookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.[28] This should not be confused with theNichols radiometer, in which the (slight) motion caused by torque (though not enough for full rotation against friction)is directly caused by light pressure.[29]As a consequence of light pressure,Einstein in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter.[30] 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 inevanescent waves momentum is transverse to direction of propagation.[31]
Historical theories about light, in chronological order
Classical Greece and Hellenism
In the fifth century BC,Empedocles postulated that everything was composed offour elements; fire, air, earth and water. He believed that goddessAphrodite 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.[32]
In about 300 BC,Euclid wroteOptica, 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.[33]
In 55 BC,Lucretius, a Roman who carried on the ideas of earlier Greekatomists, 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." (fromOn the nature of the Universe). Despite being similar to later particle theories, Lucretius's views were not generally accepted.Ptolemy (c. second century) wrote about therefraction of light in his bookOptics.[34]
Classical India
Inancient India, theHindu schools ofSamkhya andVaisheshika, 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. Theatomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.[35]TheVishnu Purana refers to sunlight as "the seven rays of the sun".[35]
The IndianBuddhists, such asDignāga in the fifth century andDharmakirti in the seventh 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.[35]
Descartes
René Descartes (1596–1650) held that light was amechanical property of the luminous body, rejecting the "forms" ofIbn al-Haytham andWitelo as well as the "species" ofRoger Bacon,Robert Grosseteste andJohannes Kepler.[36] In 1637 he published a theory of therefraction 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's theory of light is regarded as the start of modern physical optics.[36]
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's theory of theplenum. He stated in hisHypothesis of Light of 1675 that light was composed ofcorpuscles (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 thediffraction of light (which had been observed byFrancesco Grimaldi) by allowing that a light particle could create a localised wave in theaether.
Newton's theory could be used to predict thereflection of light, but could only explainrefraction by incorrectly assuming that light accelerated upon entering a densermedium because thegravitational pull was greater. Newton published the final version of his theory in hisOpticks of 1704. His reputation helped theparticle theory of light to hold sway during the eighteenth century. The particle theory of light ledPierre-Simon 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 ablack 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 inThe large scale structure of space-time, byStephen Hawking andGeorge F. R. Ellis.
The fact that light could bepolarized 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 ofcolours,Robert Hooke (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 workMicrographia ("Observation IX"). In 1672 Hooke suggested that light's vibrations could beperpendicular to the direction of propagation.Christiaan Huygens (1629–1695) worked out a mathematical wave theory of light in 1678 and published it in hisTreatise on Light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called theluminiferous aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.[37] Another supporter of the wave theory wasLeonhard Euler. He argued inNova theoria lucis et colorum (1746) thatdiffraction could more easily be explained by a wave theory.
The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 byThomas Young). Young showed by means of adiffraction experiment that light behaved as waves. He first publicly stated his "general law" of interference in January 1802, in his bookA Syllabus of a Course of Lectures on Natural and Experimental Philosophy:[38]
But the general law, by which all these appearances are governed, may be very easily deduced from the interference of two coincident undulations, which either cooperate, or destroy each other, in the same manner as two musical notes produce an alternate intension and remission, in the beating of an imperfect unison.[39]
He also proposed that different colours were caused by differentwavelengths of light and explained colour vision in terms of three-coloured receptors in the eye.
In 1816André-Marie Ampère gaveAugustin-Jean Fresnel an idea that the polarization of light can be explained by the wave theory if light were atransverse wave.[40] Later, 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 favor of the wave theory, helping to overturn Newton's corpuscular theory.[dubious –discuss] 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.[citation needed]
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 theMichelson–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, thespeed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement wasLéon Foucault, in 1850.[41] His result supported the wave theory, and the classical particle theory was finally abandoned (only to partly re-emerge in the twentieth century asphotons inquantum theory).
In 1845,Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along themagnetic field direction in the presence of a transparentdielectric, an effect now known asFaraday rotation.[42] This was the first evidence that light was related toelectromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.[42] 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.[43]
Faraday's work inspiredJames 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 inOn Physical Lines of Force. In 1873, he publishedA Treatise on Electricity and Magnetism, which contained a full mathematical description of the behavior of electric and magnetic fields, still known asMaxwell'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 andinterference. 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 aswave 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 asspectral lines).
Quantum theory
In 1900Max Planck, attempting to explainblack-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 thephotoelectric effect and suggested that these light quanta had a "real" existence. In 1923Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so calledCompton scattering) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926Gilbert N. Lewis named these light quanta particlesphotons.[44]
Eventuallyquantum mechanics came to picture light as (in some sense)both a particle and a wave, and (in another sense) as a phenomenon which isneither a particle nor a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, under some approximations light can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles) and sometimes another macroscopic metaphor (waves).
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 1924–1925,Satyendra Nath Bose showed that light followed different statistics from that of classical particles. With Einstein, they generalized this result for a whole set of integer spin particles calledbosons (after Bose) that followBose–Einstein statistics. The photon is a massless boson of spin 1.
John R. Klauder,George Sudarshan,Roy J. Glauber, andLeonard Mandel applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and thestatistics of light (seedegree of coherence). This led to the introduction of thecoherent state as a concept which addressed variations between laser light, thermal light, exoticsqueezed states, etc. as it became understood that light cannot be fully described just referring to theelectromagnetic fields describing the waves in the classical picture. In 1977,H. Jeff Kimble et al. demonstrated a single atom emitting one photon at a time, further compelling evidence that light consists of photons. Previously unknown quantum states of light with characteristics unlike classical states, such assqueezed light were subsequently discovered.
Development of short andultrashort laser pulses—created byQ switching andmodelocking techniques—opened the way to the study of what became known as ultrafast processes. Applications for solid state research (e.g.Raman spectroscopy) were found, and mechanical forces of light on matter were studied. The latter led to levitating and positioning clouds of atoms or even small biological samples in anoptical trap oroptical tweezers by laser beam. This, along withDoppler cooling andSisyphus cooling, was the crucial technology needed to achieve the celebratedBose–Einstein condensation.
Sunlight provides theenergy thatgreen plants use to createsugars mostly in the form ofstarches, which release energy into the living things that digest them. This process ofphotosynthesis provides virtually all the energy used by living things. Some species of animals generate their own light, a process calledbioluminescence. For example,fireflies use light to locate mates andvampire squid use it to hide themselves from prey.
^Pal, G.K.; Pal, Pravati (2001)."chapter 52".Textbook of Practical Physiology (1st ed.). Chennai: Orient Blackswan. p. 387.ISBN978-81-250-2021-9.Archived from the original on 8 October 2022. Retrieved11 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.ISBN978-0-262-02336-8. Retrieved11 October 2013.Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.
^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.Bibcode:1976JOSA...66..339S.doi:10.1364/JOSA.66.000339.PMID1262982.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 1,064 nm. A continuous 1,064 nm laser source appeared red, but a 1,060 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: Cambridge University Press. p. 231.ISBN978-0-521-77504-5.Archived from the original on 8 October 2022. Retrieved12 October 2013.Limits of the eye's overall range of sensitivity extends from about 310 to 1,050 nanometers
^Dash, Madhab Chandra; Dash, Satya Prakash (2009).Fundamentals of Ecology 3E. Tata McGraw-Hill Education. p. 213.ISBN978-1-259-08109-5.Archived from the original on 8 October 2022. Retrieved18 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.
^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.
^Berns, Roy S. (2019).Billmeyer and Saltzman's Principles of Color Technology. Fred W. Billmeyer, Max Saltzman (4th ed.). Hoboken, NJ:Wiley.ISBN978-1-119-36668-3.OCLC1080250734.
^Einstein, A. (1989) [1909]. "Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung" [On the development of our views concerning the nature and constitution of radiation].The Collected Papers of Albert Einstein. Vol. 2. Princeton, New Jersey: Princeton University Press. p. 391.
^Antognozzi, M.; Bermingham, C. R.; Harniman, R. L.; Simpson, S.; Senior, J.; Hayward, R.; Hoerber, H.; Dennis, M. R.; Bekshaev, A. Y. (August 2016). "Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever".Nature Physics.12 (8):731–735.arXiv:1506.04248.Bibcode:2016NatPh..12..731A.doi:10.1038/nphys3732.ISSN1745-2473.S2CID52226942.
^Singh, S. (2009).Fundamentals of Optical Engineering. Discovery Publishing House.ISBN978-81-8356-436-6.
^Ptolemy and A. Mark Smith (1996).Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary. Diane Publishing. p. 23.ISBN978-0-87169-862-9.
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