Ligeti \le-'ga-te\, Gyorgy (Sandor) (b. May 28, 1923, Diciosanmar- tin, Transylvania, Rom.) Hungarian (Transylvanian) composer. By 1950 he was teaching at the Budapest Academy, but not until he met Karlheinz Stockhausen and others in Vienna in 1956 did he find his compositional path. After a brief interest in electronic music, he gained international recognition for his avant-garde pieces for performers; these works dealt principally with shifting masses of sound and tone colours. His most famous work, Atmospheres (1961), was used in the film 2001: A Space Odyssey (1968). The opera Le Grand macabre (1978) has been widely performed in Europe.
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 x 10 14 to 7.5 x 10 14 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.
light-emitting diode (LED) Semiconductor diode that produces vis¬ ible 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 -
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optic communications systems. LEDs are formed by the so-called Ill-V compound semiconductors related to gallium arsenide. They consume little power and are long-lasting and inexpensive.
light fixture See luminaire
light-frame construction System of construction using many small and closely spaced members that can be assembled by nailing. It is the standard for U.S. suburban housing. The balloon-frame house with wood cladding, invented in Chicago in the 1840s, aided the rapid settlement of the western U.S. In North America, with its abundant softwood forests, the framed building enjoyed an extensive revival after World War II in the form of platform frames. In platform framing, each floor is framed separately, as contrasted with balloon framing, in which the studs (verti¬ cal members) extend the full height of the building. Freed from the heavy timbers of the post-and-beam system, platform framing offers ease of con¬ struction. Carpenters first fabricate a floor, which consists of wood joists and subflooring. The floor often serves as a working platform on which the stud wall frames are fabricated in sections and then lifted into place. On top of this is placed a second floor or the roof. The roof is formed of rafters (sloping joists) or wood trusses. The standard interior wall sheath¬ ing is gypsum board (drywall), which provides fire-resistance, stability, and a surface ready for finishing. Framed structures traditionally were constructed individually at each house site; today many of the framing elements are mass-produced elsewhere and assembled on-site.
light quantum See photon
light-year 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.
Lightfoot, Gordon (b. Nov. 17, 1938, Orillia, Ont., Can.) Canadian singer and songwriter. He began writing folk-oriented pop singles in the mid-1960s, including “Early Morning Rain” and “Ribbon of Darkness.” His later hits include “If You Could Read My Mind” and “The Wreck of the Edmund Fitzgerald .” His songs have been covered by singers rang¬ ing from Barbra Streisand to Jerry Lee Lewis.
lighthouse Structure, usually with a tower, built onshore or on the sea¬ bed to signal danger or provide aid to seafarers. The first known light¬ house was the Pharos of Alexandria. The modern lighthouse dates only from the early 18th century. Initially made of wood, these towers were often washed away in severe storms. The first lighthouse made of inter¬ locking masonry blocks was built on the treacherous Eddystone Rocks reef, off Plymouth, England (1759). Interlocking masonry blocks remained the principal material of lighthouse construction until they were replaced by concrete and steel in the 20th century. Modern construction methods have facilitated the building of offshore lighthouses. The most common illuminant is the electric-filament lamp. Refinements in lenses (e.g., the Fresnel lens) and reflectors made it possible to substantially increase the light’s intensity. Radio and satellite-based navigation systems have greatly reduced the need for large lighthouses in sighting land.
lighting Use of an artificial source of light for illumination. It is a key element of architecture and interior design. Residential lighting uses mainly either incandescent lamps or fluorescent lamps and often depends heavily on movable fixtures plugged into outlets; built-in lighting is typi¬ cally found in kitchens, bathrooms, and corridors and in the form of hang¬ ing pendants in dining rooms and sometimes recessed fixtures in living rooms. Lighting in nonresidential buildings is predominantly fluorescent. High-pressure sodium-vapor lamps (see electric discharge lamp) have higher efficiency and are used in industrial applications. Halogen lamps have resi¬ dential, industrial, and photographic applications. Depending on their fix¬ tures, lamps (bulbs) produce a variety of lighting conditions. Incandescent lamps placed in translucent glass globes create diffuse effects; in recessed ceiling-mounted fixtures with reflectors, they can light walls or floors evenly. Fluorescent fixtures are typically recessed and rectangular, with prismatic lenses, but other types including indirect cove lights (see coving) and luminous ceilings, in which lamps are placed above suspended trans¬ lucent panels. Mercury-vapor and high-pressure sodium-vapor lamps are placed in simple reflectors in industrial spaces, in pole-mounted streetlight fixtures, and in indirect up-lighting fixtures for commercial applications.
lightning Visible discharge of electricity when part of the atmosphere acquires enough electrical charge to overcome the resistance of the air. During a thunderstorm, lightning flashes can occur within clouds, between
clouds, between clouds and air, or from clouds to the ground. Lightning is usually associated with cumulonimbus clouds (thunderclouds) but also occurs in nimbostratus clouds, in snowstorms and dust storms, and some¬ times in the dust and gases emitted by a volcano. A typical lightning flash involves a potential difference between cloud and ground of several hun¬ dred million volts. Temperatures in the lightning channel are on the order of 30,000 K (50,000 °F). A cloud-to-ground flash comprises at least two strokes: a pale leader stroke that strikes the ground and a highly luminous return stroke. The leader stroke reaches the ground in about 20 millisec¬ onds; the return stroke reaches the cloud in about 70 microseconds. The thunder associated with lightning is caused by rapid heating of air along the length of the lightning channel. The heated air expands at supersonic speeds. The shock wave decays within a metre or two into a sound wave, which, modified by the intervening air and topography, produces a series of rumbles and claps. See also thunderstorm.