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light, visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm. The term “light” is often extended to adjacent wavelength ranges that the eye cannot detect—to infrared radiation, which has a frequency less than that of visible light, and to ultraviolet radiation and black light, which have a frequency greater than that of visible light.

If white light, which contains all visible wavelengths, is separated, or dispersed, into a spectrum, each wavelength is seen to correspond to a different color. Light that is all of the same wavelength and phase (all the waves are in step with one another) is called “coherent”; one of the most important modern applications of light has been the development of a source of coherent light—the laser.

The Nature of Light

The scientific study of the behavior of light is called optics and covers reflection of light by a mirror or other object, refraction by a lens or prism, diffraction of light as it passes by the edge of an opaque object, and interference patterns resulting from diffraction. Also studied is the polarization of light. Any successful theory of the nature of light must be able to explain these and other optical phenomena.

The Wave, Particle, and Electromagnetic Theories of Light

The earliest scientific theories of the nature of light were proposed around the end of the 17th cent. In 1690, Christian Huygens proposed a theory that explained light as a wave phenomenon. However, a rival theory was offered by Sir Isaac Newton in 1704. Newton, who had discovered the visible spectrum in 1666, held that light is composed of tiny particles, or corpuscles, emitted by luminous bodies. By combining this corpuscular theory with his laws of mechanics, he was able to explain many optical phenomena.

For more than 100 years, Newton's corpuscular theory of light was favored over the wave theory, partly because of Newton's great prestige and partly because not enough experimental evidence existed to provide an adequate basis of comparison between the two theories. Finally, important experiments were done on the diffraction and interference of light by Thomas Young (1801) and A. J. Fresnel (1814–15) that could only be interpreted in terms of the wave theory. The polarization of light was still another phenomenon that could only be explained by the wave theory. Thus, in the 19th cent. the wave theory became the dominant theory of the nature of light.

The wave theory received additional support from the electromagnetic theory of James Clerk Maxwell (1864), who showed that electric and magnetic fields were propagated together and that their speed was identical with the speed of light. It thus became clear that visible light is a form of electromagnetic radiation, constituting only a small part of the electromagnetic spectrum. Maxwell's theory was confirmed experimentally with the discovery of radio waves by Heinrich Hertz in 1886.

Modern Theory of the Nature of Light

With the acceptance of the electromagnetic theory of light, only two general problems remained. One of these was that of the luminiferous ether, a hypothetical medium suggested as the carrier of light waves, just as air or water carries sound waves. The ether was assumed to have some very unusual properties, e.g., being massless but having high elasticity. A number of experiments performed to give evidence of the ether, most notably by A. A. Michelson in 1881 and by Michelson and E. W. Morley in 1887, failed to support the ether hypothesis. With the publication of the special theory of relativity in 1905 by Albert Einstein, the ether was shown to be unnecessary to the electromagnetic theory.

The second main problem, and the more serious of the two, was the explanation of various phenomena, such as the photoelectric effect, that involved the interaction of light with matter. Again the solution to the problem was proposed by Einstein, also in 1905. Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 to cover not only vibrations of the source of radiation but also vibrations of the radiation itself. He thus suggested that light, and other forms of electromagnetic radiation as well, travel as tiny bundles of energy called light quanta, or photons. The energy of each photon is directly proportional to its frequency.

With the development of the quantum theory of atomic and molecular structure by Niels Bohr and others, it became apparent that light and other forms of electromagnetic radiation are emitted and absorbed in connection with energy transitions of the particles of the substance radiating or absorbing the light. In these processes, the quantum, or particle, nature of light is more important than its wave nature. When the transmission of light is under consideration, however, the wave nature dominates over the particle nature. In 1924, Louis de Broglie showed that an analogous picture holds for particle behavior, with moving particles having certain wavelike properties that govern their motion, so that there exists a complementarity between particles and waves known as particle-wave duality (see also complementarity principle). The quantum theory of light has successfully explained all aspects of the behavior of light.

The Speed of Light

An important question in the history of the study of light has been the determination of its speed and of the relationship of this speed to other physical phenomena. At one time it was thought that light travels with infinite speed—i.e., it is propagated instantaneously from its source to an observer. Olaus Rømer showed that it was finite, however, and in 1675 estimated its value from differences in the time of eclipse of certain of Jupiter's satellites when observed from different points in the earth's orbit. More accurate measurements were made during the 19th cent. by A. H. L. Fizeau (1849), using a toothed wheel to interrupt the light, and by J. B. L. Foucault (1850), using a rotating mirror. The most accurate measurements of this type were made by Michelson. Modern electronic methods have improved this accuracy, yielding a value of 2.99792458 × 108 m (c.186,000 mi) per sec for the speed of light in a vacuum, and less for its speed in other media. The theory of relativity predicts that the speed of light in a vacuum is the limiting velocity for material particles; no particle can be accelerated from rest to the speed of light, although it may approach it very closely. Particles moving at less than the speed of light in a vacuum but greater than that of light in some other medium will emit a faint blue light known as Cherenkov radiation when they pass through the other medium. This phenomenon has been used in various applications involving elementary particles.

Luminous and Illuminated Bodies

In general, vision is due to the stimulation of the optic nerves in the eye by light either directly from its source or indirectly after reflection from other objects. A luminous body, such as the sun, another star, or a light bulb, is thus distinguished from an illuminated body, such as the moon and most of the other objects one sees. The amount and type of light given off by a luminous body or reflected by an illuminated body is of concern to the branch of physics known as photometry (see also lighting). Illuminated bodies not only reflect light but sometimes also transmit it. Transparent objects, such as glass, air, and some liquids, allow light to pass through them. Translucent objects, such as tissue paper and certain types of glass, also allow light to pass through them but diffuse (scatter) it in the process, so that an observer cannot see a clear image of whatever lies on the other side of the object. Opaque objects do not allow light to pass through them at all. Some transparent and translucent objects allow only light of certain wavelengths to pass through them and thus appear colored. The colors of opaque objects are caused by selective reflection of certain wavelengths and absorption of others.


See W. L. Bragg, The Universe of Light (1959); J. Rublowsky, Light (1964); H. Haken, Light (1981).

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The term light, as commonly used, refers to the kind of radiant electromagnetic energy that is associated with vision. In a broader sense, light includes the entire range of radiation known as the electromagnetic spectrum. The branch of science dealing with light, its origin and propagation, its effects, and other phenomena associated with it is called optics. Spectroscopy is the branch of optics that pertains to the production and investigation of spectra. See Optics, Spectroscopy

Principal effects

The electromagnetic spectrum is a broad band of radiant energy that extends over a range of wavelengths running from trillionths of inches to hundreds of miles; wavelengths of visible light are measured in hundreds of thousandths of an inch. Arranged in order of increasing wavelength, the radiation making up the electromagnetic spectrum is termed gamma rays, x-rays, ultraviolet rays, visible light, infrared waves, microwaves, radio waves, and very long electromagnetic waves. See Electromagnetic radiation

The fact that light travels at a finite speed or velocity is well established. In round numbers, the speed of light in vacuum or air may be said to be 186,000 mi/s or 300,000 km/s. Measurements of the speed of light, c, which had attracted physicists for 308 years, came to an end in 1983 when the new definition of the meter fixed the value of the speed of light. Highly precise values of c were obtained by extending absolute frequency measurements into a region of the electromagnetic spectrum where wavelengths can be most accurately measured. These advances were facilitated by the use of stabilized lasers and high-speed tungsten-nickel diodes which were used to measure the lasers' frequencies. The measurements of the speed of light and of the frequency of lasers yielded a value of the speed of light limited only by the standard of length which was then in use. This permitted a redefinition of the meter in which the value of the speed of light assumed an exact value, 299,792,458 m/s. The meter is defined as the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. See Laser

One of the most easily observed facts about light is its tendency to travel in straight lines. Careful observation shows, however, that a light ray spreads slightly when passing the edges of an obstacle. This phenomenon is called diffraction. The reflection of light is also well known. Reflection of light from smooth optical surfaces occurs so that the angle of reflection equals the angle of incidence, a fact that is most readily observed with a plane mirror. When light is reflected irregularly and diffusely, the phenomenon is termed scattering. The scattering of light by gas particles in the atmosphere causes the blue color of the sky. See Diffraction, Reflection of electromagnetic radiation

The type of bending of light rays called refraction is caused by the fact that light travels at different speeds in different media—faster, for example, in air than in either glass or water. Refraction occurs when light passes from one medium to another in which it moves at a different speed. Familiar examples include the change in direction of light rays in going through a prism, and the bent appearance of a slick partially immersed in water. See Refraction of waves

In the phenomenon called interference, rays of light emerging from two parallel slits combine on a screen to produce alternating light and dark bands. This effect can be obtained quite easily in the laboratory, and is observed in the colors produced by a thin film of oil on the surface of a pool of water. Polarization of light is usually shown with Polaroid disks. Such disks are quite transparent individually. When two of them are placed together, however, the degree of transparency of the combination depends upon the relative orientation of the disks. It can be varied from ready transmission of light to almost total opacity, simply by rotating one disk with respect to the other. See Interference of waves, Polarized light

When light is absorbed by certain substances, chemical changes take place. This fact forms the basis for the science of photochemistry.


Phenomena involving light may be classed into three groups: electromagnetic wave phenomena, corpuscular or quantum phenomena, and relativistic effects. The relativistic effects appear to influence similarly the observation of both corpuscular and wave phenomena. See Relativity

Wave phenomena

Interference and diffraction are the most striking manifestations of the wave character of light. Their fundamental similarity can be demonstrated in a number of experiments. The wave aspect of the entire spectrum of electromagnetic radiation is most convincingly shown by the similarity of diffraction pictures produced on a photographic plate, placed at some distance behind a diffraction grating, by radiations of different frequencies, such as x-rays and visible light. The interference phenomena of light are, moreover, very similar to interference of electronically produced microwaves and radio waves.

Polarization demonstrates the transverse character of light waves. Further proof of the electromagnetic character of light is found in the possibility of inducing, in a transparent body that is being traversed by a beam of plane-polarized light, the property of rotating the plane of polarization of the beam when the body is placed in a magnetic field. See Faraday effect

The fact that the velocity of light had been calculated from electric and magnetic parameters (permittivity and permeability) was at the root of Maxwell's conclusion in 1865 that “light, including heat and other radiations if any, is a disturbance in the form of waves propagated…according to electromagnetic laws.” Finally, the observation that electrons and neutrons can give rise to diffraction patterns quite similar to those produced by visible light has made it necessary to ascribe a wave character to particles. See Electron diffraction, Neutron diffraction

Corpuscular phenomena

In its interactions with matter, light exchanges energy only in discrete amounts, called quanta. This fact is difficult to reconcile with the idea that light energy is spread out in a wave, but is easily visualized in terms of corpuscles, or photons, of light.

The radiation from theoretically perfect heat radiators, called blackbodies, involves the exchange of energy between radiation and matter in an enclosed cavity. The observed frequency distribution of the radiation emitted by the enclosure at a given temperature of the cavity can be correctly described by theory only if one assumes that light of frequency ν is absorbed in integral multiples of a quantum of energy equal to hν, where h is a fundamental physical constant called Planck's constant.

When a monochromatic beam of electromagnetic radiation illuminates the surface of a solid (or less commonly, a liquid), electrons are ejected from the surface in the phenomenon known as photoemission or the external photoelectric effect. It is found that the emission of these photoelectrons, as they are called, is immediate, and independent of the intensity of the light beam, even at very low light intensities. This fact excludes the possibility of accumulation of energy from the light beam until an amount corresponding to the kinetic energy of the ejected electron has been reached.

The scattering of x-rays of frequency ν0 by the lighter elements is caused by the collision of x-ray photons with electrons. Under such circumstances, both a scattered x-ray photon and a scattered electron are observed, and the scattered x-ray has a lower frequency than the impinging x-ray. The kinetic energy of the impinging x-ray, the scattered x-ray, and the scattered electron, as well as their relative directions, are in agreement with calculations involving the conservation of energy and momentum. See Compton effect, Heat radiation, Photon

Quantum theories

The need for reconciling Maxwell's theory of the electromagnetic field, which describes the electromagnetic wave character of light, with the particle nature of photons, which demonstrates the equally important corpuscular character of light, has resulted in the formulation of several theories which go a long way toward giving a satisfactory unified treatment of the wave and the corpuscular picture. These theories incorporate, on one hand, the theory of quantum electrodynamics, first set forth by P. A. M. Dirac, P. Jordan, W. Heisenberg, and W. Pauli, and on the other, the earlier quantum mechanics of L. de Broglie, Heisenberg, and E. Schrödinger. Unresolved theoretical difficulties persist, however, in the higher-than-first approximations of the interactions between light and elementary particles.

Dirac's synthesis of the wave and corpuscular theories of light is based on rewriting Maxwell's equations in a Hamiltonian form resembling the Hamiltonian equations of classical mechanics. Using the same formalism involved in the transformation of classical into wave-mechanical equations by the introduction of the quantum of action hν, Dirac obtained a new equation of the electromagnetic field. The solutions of this equation require quantized waves, corresponding to photons. The superposition of these solutions represents the electromagnetic field. The quantized waves are subject to Heisenberg's uncertainty principle. The quantized description of radiation cannot be taken literally in terms of either photons or waves, but rather is a description of the probability of occurrence in a given region of a given interaction or observation. See Hamilton's equations of motion, Quantum electrodynamics, Quantum field theory, Quantum mechanics, Relativistic quantum theory, Uncertainty principle

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


Electromagnetic radiation to which the human eye is sensitive; light is thus also called visible or optical radiation. The wavelength range varies from person to person but usually lies within the limits of 380 to 750 nanometers (daytime). Light therefore forms a very narrow band of the electromagnetic spectrum between the infrared and ultraviolet bands. As the wavelength decreases, the color of the light changes through the spectral hues of red, orange, yellow, green, blue, indigo, and violet. These colors, which may be seen in a rainbow or produced by a prism, form the visible spectrum. In daylight, the eye is most sensitive to greenish-yellow light of wavelength about 550 nm; the dark-adapted eye has peak sensitivity around 510 nm, the range extending to about 620 nm. Optical astronomy covers wavelengths from about 300 to 900 nm, i.e. slightly wider than the human range. The Earth's atmosphere is transparent to light so that astronomy at optical wavelengths has not experienced the problems with detection and observation that have bedeviled studies at other wavelengths. Atmospheric turbulence does, however, adversely affect ground-based optical astronomy, as do clouds and precipitation. See atmospheric windows. See also light pollution.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006


An opening through which daylight is admitted to the interior space of a building; a pane of glass, window, or compartment of a window.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved


(religion, spiritualism, and occult)

Light is the oldest British Spiritualist journal, founded in 1881 by William Stainton Moses and Dawson Rogers. It was issued weekly. In recent years it was issued as the official journal of the College of Psychic Studies, London, England.


Fodor, Nandor: Encyclopedia of Psychic Science. London: Arthurs Press, 1933
The Spirit Book © 2006 Visible Ink Press®. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(1) In the narrow sense, light is visible radiation, that is, electromagnetic waves in the range of frequencies perceived by the human eye. This range extends from 7.5 × 1014 to 4.3 × 1014 hertz (Hz) and corresponds to the range of wavelengths in a vacuum from 400 to 700 nanometers (nm). The eye is able to detect light of very high intensity in a somewhat broader frequency range. The spectral sensitivity of the average human eye—that is, the dependence of the eye’s sensitivity to light on the frequency of the light—is characterized by the spectral luminous efficiency curve, or luminosity curve. Calculations in illuminating engineering are based on this curve. In the general case, but not in every individual case, differences in light-wave frequencies or sets of frequencies are peceived by man as differences in color.

(2) In the broad sense, light is optical radiation and includes radiation in the visible, ultraviolet, and infrared regions of the spectrum. The range of frequencies in this case extends from approximately 3 × 1011 Hz to approximately 3 × 1017 Hz; the corresponding range of wavelengths in a vacuum is from 1 mm to 1 nm. In this optical range the physical properties of the radiation share many common features, and the same methods can to a large extent be used to investigate radiation throughout the range (see). In particular, it is in the optical range that the wave and corpuscular properties of electromagnetic radiation begin to show up simultaneously in a clear manner.

The principal phenomena characteristic of light and of the processes of the interaction of light with matter are discussed in various articles of the encyclopedia, including DIFFRACTION OF LIGHT, INTERFERENCE OF LIGHT, CRYSTAL OPTICS, MAGNETO-OPTICS, METAL OPTICS, OPTICAL ACTIVITY, REFLECTION OF LIGHT, ABSORPTION OF LIGHT, REFRACTION OF LIGHT, POLARIZATION OF LIGHT, AND SCATTERING OF LIGHT.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

What does it mean when you dream about a light?

Illumination in a dream can symbolize the shedding of light on a situation or problem, enlightenment of one’s consciousness, and lighting the way on a physical or spiritual journey. The extinguishing of light may signify the end of an old situation. Spiritually, this is a very positive symbol.

The Dream Encyclopedia, Second Edition © 2009 Visible Ink Press®. All rights reserved.


Electromagnetic radiation with wavelengths capable of causing the sensation of vision, ranging approximately from 400 (extreme violet) to 770 nanometers (extreme red). Also known as light radiation; visible radiation.
More generally, electromagnetic radiation of any wavelength; thus, the term is sometimes applied to infrared and ultraviolet radiation.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. An aperture through which daylight is admitted to the interior of a building.
2. A pane of glass, a window, or a compartment of a window.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


god of light. [Gk. Myth.: Espy, 28]
twin gods of light. [Hindu Myth.: Bent, 60]
god of light and peace. [Norse Myth.: Leach, 106]
Jesus Christ “I
am the light of the world.” [N.T.: John 8:12]
god of light. [Pers. Myth.: Wheeler, 246]
patée cross
four spear-headed arms; symbolizes solar light. [Christian Iconog.: Brewer Dictionary, 280; Jobes, 386]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.


electromagnetic radiation that is capable of causing a visual sensation and has wavelengths from about 380 to about 780 nanometres


1. having relatively low density
2. relatively low in alcoholic content
3. (of an industry) engaged in the production of small consumer goods using light machinery
4. Aeronautics (of an aircraft) having a maximum take-off weight less than 5670 kilograms (12 500 pounds)
5. Chem (of an oil fraction obtained from coal tar) having a boiling range between about 100? and 210?C
6. (of a railway) having a narrow gauge, or in some cases a standard gauge with speed or load restrictions not applied to a main line
7. Bridge
a. (of a bid) made on insufficient values
b. (of a player) having failed to take sufficient tricks to make his contract
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


LIfecycle Global HyperText.

A project in the CERN ECP/TP group whereby documents resulting from the software life cycle are available as hypertext.
This article is provided by FOLDOC - Free Online Dictionary of Computing (

visible light

The part of the electromagnetic spectrum that humans perceive. The infrared and ultraviolet bands precede and follow visible light. See visible light communication.

Visible Light
Our eyes perceive a tiny sliver of the electromagnetic spectrum. The wavelengths from (approximately) 400 to 750 nanometers provide us with our view of the physical universe.

Visible Light
Our eyes perceive a tiny sliver of the electromagnetic spectrum. The wavelengths from (approximately) 400 to 750 nanometers provide us with our view of the physical universe.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.


Whether you are dreaming or awake, light is a very positive symbol. It represents enlightenment, lifting of shadow, the acquisition of understanding and knowledge, and a positive force. Depending on the details of the dream, you may give the light spiritual or physical meaning. For some, the light may represent a higher force or power, God’s presence, or rays of His love and peace. Others may find this dream reassuring as it may represent a well-lit journey through life, or simply a solution to a current problem. Light always refers to consciousness.
Bedside Dream Dictionary by Silvana Amar Copyright © 2007 by Skyhorse Publishing, Inc.