light, visible electromagnetic radiation electromagnetic radiation, energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an
..... Click the link for more information. . Of the entire electromagnetic spectrum spectrum, arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass
..... Click the link for more information. , 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 infrared radiation, electromagnetic radiation having a wavelength in the range from c.75 × 10⁻⁶ cm to c.100,000 × 10⁻⁶ cm (0.000075–0.1 cm).
..... Click the link for more information. , which has a frequency less than that of visible light, and to ultraviolet radiation UV index predicts how long it would take a light-skinned American to get a sunburn if exposed, unprotected, to the noonday sun, given the geographical location and the local weather.
..... Click the link for more information. 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 color, effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the
..... Click the link for more information. . 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 laser [acronym for light amplification by stimulated emission of radiation], device for the creation, amplification, and transmission of a narrow, intense beam of coherent light .
..... Click the link for more information. .

The Nature of Light

The scientific study of the behavior of light is called optics optics, scientific study of light . Physical optics is concerned with the genesis, nature, and properties of light; physiological optics with the part light plays in vision ; and geometrical optics with the reflection and refraction of light as encountered in the
..... Click the link for more information. and covers reflection reflection, return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the reflection of light and sound are similar, for both normally travel in straight lines and both are wave phenomena.
..... Click the link for more information. of light by a mirror mirror, in optics, a reflecting surface that forms an image of an object when light rays coming from that object fall upon it (see reflection ). Usually mirrors are made of plate glass, one side of which is coated with metal or some special preparation to serve as a
..... Click the link for more information. or other object, refraction refraction, in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass.
..... Click the link for more information. by a lens lens, device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved.
..... Click the link for more information. or prism prism, in optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular.
..... Click the link for more information. , diffraction of light as it passes by the edge of an opaque object, and interference interferometer. When the wavelength of the light is known, the interferometer indicates the thickness of the film by the interference patterns it forms. The reverse process, i.e., the measurement of the length of an unknown light wave, can also be carried out by the interferometer.
..... Click the link for more information. patterns resulting from diffraction. Also studied is the polarization of light polarization of light, orientation of the vibration pattern of light waves in a singular plane.

Characteristics of Polarization

Polarization is a phenomenon peculiar to transverse waves, i.e.
..... Click the link for more information. . 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 Huygens, Christiaan (krĭs`tyän hoi`gəns), 1629–95, Dutch mathematician and physicist; son of Constantijn Huygens.
..... Click the link for more information. proposed a theory that explained light as a wave wave, in physics, the transfer of energy by the regular vibration , or oscillatory motion, either of some material medium or by the variation in magnitude of the field vectors of an electromagnetic field (see electromagnetic radiation ).
..... Click the link for more information. phenomenon. However, a rival theory was offered by Sir Isaac Newton Newton, Sir Isaac, 1642–1727, English mathematician and natural philosopher (physicist), who is considered by many the greatest scientist that ever lived.
..... Click the link for more information. 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 ether or aether, in physics and astronomy, a hypothetical medium for transmitting light and heat (radiation), filling all unoccupied space; it is also called luminiferous ether. In Newtonian physics all waves are propagated through a medium, e.g.
..... Click the link for more information. , 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 relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. 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 photoelectric effect, emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887.
..... Click the link for more information. , 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 quantum theory, modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. 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 photon (fō`tŏn), the particle composing light and other forms of electromagnetic radiation , sometimes called light quantum.
..... Click the link for more information. . 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 complementarity principle, physical principle enunciated by Niels Bohr in 1928 stating that certain physical concepts are complementary. If two concepts are complementary, an experiment that clearly illustrates one concept will obscure the other complementary one.
..... Click the link for more information. ). 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 Cherenkov radiation or Cerenkov radiation [for P. A. Cherenkov ], light emitted by a transparent medium when charged particles pass through it at a speed greater than the speed of light in the medium.
..... Click the link for more information. when they pass through the other medium. This phenomenon has been used in various applications involving elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. .

Luminous and Illuminated Bodies

In general, vision vision, physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived.

Vision in Humans

The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive
..... Click the link for more information. 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 photometry (fōtŏm`ətrē)
..... Click the link for more information. (see also lighting lighting, light produced by artificial means to allow visibility in enclosures and at night. For stage lighting, see scene design and stage lighting .

Early Sources of Artificial Lighting

..... Click the link for more information. ). 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.

Bibliography

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

light

That portion of the electromagnetic spectrum visible to the human eye. It ranges from the red end to the violet end of the spectrum, with wavelengths from 700 to 400 nanometres and frequencies from 4.3 × 10¹⁴ to 7.5 × 10¹⁴ Hz. Like all electromagnetic radiation, it travels through empty space at a speed of about 186,000 mi/sec (300,000 km/sec). In the mid-19th century, light was described by James Clerk Maxwell in terms of electromagnetic waves, but 20th-century physicists showed that it exhibits properties of particles as well; its carrier particle is the photon. Light is the basis for the sense of sight and for the perception of colour. See also optics; wave-particle duality.

See visible light.

Light

Apollo

god of light. [Gk. Myth.: Espy, 28]

Asvins

twin gods of light. [Hindu Myth.: Bent, 60]

Balder

god of light and peace. [Norse Myth.: Leach, 106]

Jesus Christ “I

am the light of the world.” [N.T.: John 8:12]

Mithras

god of light. [Pers. Myth.: Wheeler, 246]

patée cross

four spear-headed arms; symbolizes solar light. [Christian Iconog.: Brewer Dictionary, 280; Jobes, 386]

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