zodiacal light or zodiacal band, a faint band of light sometimes seen in the western sky just after sunset in the spring, extending up from the horizon at the point where the sun has just set, or in the eastern sky just before sunrise in the autumn. The light is so faint as to be obscured by moonlight. It is caused by the reflection and scattering of sunlight by a sparse band of tiny dust particles that appears to be an extension of the solar corona, stretching out beyond the orbit of Jupiter. Concentrated in the plane of the ecliptic, the faint light is best seen in the region of the sky called the zodiac. Near the equator the zodiacal light sometimes seems to stretch completely across the sky. It was first investigated and explained by the astronomer Gian Domenico Cassini about 1690.

See also gegenschein.

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., waves that vibrate in a direction perpendicular to their direction of propagation. Light is a transverse electromagnetic wave (see electromagnetic radiation). Thus a light wave traveling forward can vibrate up and down (in the vertical plane), from side to side (in the horizontal plane), or in an intermediate direction. Ordinarily a ray of light consists of a mixture of waves vibrating in all the directions perpendicular to its line of propagation. If for some reason the vibration remains constant in direction, the light is said to be polarized.

It is found, for example, that reflected light is always polarized to some extent. Light can also be polarized by double refraction. Any transparent substance has the property of refracting or bending a ray of light that enters it from outside. Certain crystals, however, such as calcite (Iceland spar), have the property of refracting unpolarized incident light in two different directions, thus splitting an incident ray into two rays. It is found that the two refracted rays (the ordinary ray and the extraordinary ray) are both polarized and that their directions of polarization are perpendicular to each other. This occurs because the speed of the light in the crystal—hence the angle at which the light is refracted—varies with the direction of polarization. Unpolarized incident light can be regarded as a mixture of two different polarization states separated into two components by the crystal. (In most substances the speed of light is the same for all directions of polarization, and no separation occurs.)

Polarization Techniques

Unpolarized light can be converted into a single polarized beam by means of the Nicol prism, a device that separates incident light into two rays by double refraction; the unwanted ray is removed from the beam by reflection. Polarized light can also be produced by using a tourmaline crystal. Tourmaline (a double-refracting substance) removes one of the polarized rays by absorption. Another commonly used polarizer consists of a sheet of transparent material in which are embedded many tiny polarizing crystals.

Any system by which light is polarized in a particular direction is transparent only to light polarized in that direction. Thus, when originally unpolarized light passes successively through two polarizers whose directions of polarization are mutually perpendicular the light is completely blocked; light transmitted by the first polarizer is polarized and is stopped by the second. If the second polarizer is rotated so that the directions of polarization are no longer perpendicular, the amount of light transmitted gradually increases, becoming brightest when the polarizers are exactly aligned. This property is used in various light filter combinations.

A number of substances can polarize light in other ways than in one plane, causing what are called circular polarization or elliptical polarization, for example. Organic substances that affect polarized light that passes through their solution are called optically active. In certain acids and other solutions the plane of polarized light is rotated to either the right or the left; their activity is usually indicated by the prefix dextro- or d- if the rotation is to the right and by levo-, laevo-, or l- if the rotation is to the left.

The instrument used to determine in which direction this optical rotation occurs is called a polariscope. A very simple form consists essentially of two crystals of some polarizing substance such as tourmaline. The solution to be tested is placed between them. Light is then directed through the first crystal, or polarizer, and is plane-polarized. After passing through the solution its plane is rotated; the direction and the degree of rotation are indicated by the position in which the second crystal must be placed to permit passage of the light that has gone through the solution. The polarimeter is a polariscope that measures the amount of rotation; when used for sugar solutions it is commonly called a saccharimeter.

light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 10¹² km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant. See also parsec.

light-emitting diode: see diode.

light horse, any breed of horse that is used primarily for riding or for light work such as pulling buggies. Light horses have their origin in the Middle East and N Africa. All modern breeds of light horse trace their origins to the Arabian horse, usually through the Thoroughbred. Light horses are classified according to training, e.g., racers, trotters, riding horses, and cow horses. See also pony, American saddlebred horse, Appaloosa, Morgan, palomino, Pinto, quarter horse, and Standardbred.

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.

electric-light bug: see water bug.

black light: see ultraviolet radiation.

Lee, Light-Horse Harry: see Lee, Henry.

Drummond light: see calcium oxide.

Band of very faint light in the night sky. It is thought to be sunlight reflected from interplanetary dust grains lying mostly in the plane of the zodiac, or ecliptic. Seen in the west after twilight and in the east before dawn, it is most clearly visible in the tropics, where the ecliptic is approximately perpendicular to the horizon. In midnorthern latitudes it is best seen evenings in February and March and mornings in September and October (vice versa in midsouthern latitudes). The light can be followed visually to a point about 90° from the Sun. It continues to the region opposite the Sun, where a slight enhancement, the gegenschein, is visible.

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Portion of the electromagnetic spectrum extending from the violet end of visible light to the X-ray region. Ultraviolet (UV) radiation lies between wavelengths of about 400 nanometres and 10 nanometres, corresponding to frequencies of 7.5 × 10¹⁴ Hz to 3 × 10¹⁶ Hz. Most UV rays from the Sun are absorbed by the Earth's ozone layer. UV has low penetrating power, so its effects on humans are limited to the skin. These effects include stimulation of production of vitamin D, sunburn, suntan, aging signs, and carcinogenic changes. UV radiation is also used to treat jaundice in newborns, to sterilize equipment, and to produce artificial light.

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Distance traveled by light moving in a vacuum in one year, at its accepted speed of 186,282 mi/second (299,792 km/second). It equals about 5.9 trillion mi (9.5 trillion km), 63,240 astronomical units, or 0.307 parsec.

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Semiconductor diode that produces visible or infrared light when subjected to an electric current, as a result of electroluminescence. Visible-light LEDs are used in many electronic devices as indicator lamps (e.g., an on/off indicator) and, when arranged in a matrix, to spell out letters or numbers on alphanumeric displays. Infrared LEDs are used in optoelectronics (e.g., in auto-focus cameras and television remote controls) and as light sources in some long-range fibre-optic communications systems. LEDs are formed by the so-called III-V compound semiconductors related to gallium arsenide. They consume little power and are long-lasting and inexpensive.

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or light quantum

Minute energy packet of electromagnetic radiation. In 1900 Max Planck found that heat radiation is emitted and absorbed in distinct units, which he called quanta. In 1905 Albert Einstein explained the photoelectric effect, proposing the existence of discrete energy packets in light. The term photon came into use for these packets in 1926. The energies of photons range from high-energy gamma rays and X rays to low-energy infrared and radio waves, though all travel at the same speed, the speed of light. Photons have no electric charge or rest mass and are the carriers of the electromagnetic field.

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Device that produces an intense beam of coherent light (light composed of waves having a constant difference in phase). Its name, an acronym derived from “light amplification by stimulated emission of radiation,” describes how its beam is produced. The first laser, constructed in 1960 by Theodore Maiman (born 1927) based on earlier work by Charles H. Townes, used a rod of ruby. Light of a suitable wavelength from a flashlight excited (see excitation) the ruby atoms to higher energy levels. The excited atoms decayed swiftly to slightly lower energies (through phonon reactions) and then fell more slowly to the ground state, emitting light at a specific wavelength. The light tended to bounce back and forth between the polished ends of the rod, stimulating further emission. The laser has found valuable applications in microsurgery, compact-disc players, communications, and holography, as well as for drilling holes in hard materials, alignment in tunnel drilling, long-distance measurement, and mapping fine details.

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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. Seealso optics; wave-particle duality.

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(born Jan. 29, 1756, Prince William county, Va.—died March 25, 1818, Cumberland Island, Ga., U.S.) American army officer and politician. In the American Revolution he rose to cavalry commander (earning the nickname “Light-Horse Harry”) and led victories at Paulus Hook, N.J., and in the South. As governor of Virginia (1791–94), he commanded the army that suppressed the Whiskey Rebellion (1794). In the U.S. House of Representatives (1799–1801), he wrote the resolution eulogizing George Washington as “first in war, first in peace, and first in the hearts of his countrymen.” After 1800 Lee failed in several land and financial speculations and was twice imprisoned for debt. He was the father of Robert E. Lee.

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