Definitions

effect

tunneling

[tuhn-l]
or barrier penetration

In physics, the passage of a particle through a seemingly impassable energy barrier. Though a particle's energy may be too low to surmount a barrier in classical physics, the particle may still cross the barrier as a consequence of its quantum-mechanical wave properties. An important application of this phenomenon is in the operation of the scanning tunneling microscope.

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Artificial imitation of sound to accompany action and supply realism in a dramatic production. Sound effects were first used in the theatre, where they can represent a range of action too vast or difficult to present onstage, from battles and gunshots to trotting horses and rainstorms. Various methods were devised by backstage technicians to reproduce sounds (e.g., rattling sheet metal to create thunder); today most sound effects are reproduced by recordings. An important part of old-fashioned radio dramas, sound effects are still painstakingly added to television and movie soundtracks.

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Process in which two dissimilar materials in close contact act as an electric cell when struck by light or other radiant energy. In crystals of certain elements, such as silicon and germanium, the electrons are usually not free to move from atom to atom. Light striking the crystal provides the energy needed to free electrons from their bound condition. These electrons can cross the junction between two dissimilar crystals more easily in one direction than another, so one side of the junction acquires a negative voltage with respect to the other. As long as light falls on the two materials, the photovoltaic battery can continue to provide voltage and current. The current can be used to measure the brightness of the light or as a source of power, as in a solar cell.

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Phenomenon in which charged particles are released from a material when it absorbs radiant energy (see radiation). It is often thought of as the ejection of electrons from the surface of a metal plate when visible light falls on it. It can also occur if the radiation is in the wavelength range of ultraviolet radiation, X rays, or gamma rays. The emitting surface may be a solid, liquid, or gas, and the emitted particles may be electrons or ions. The effect was discovered in 1887 by Heinrich Hertz and explained by Albert Einstein in work for which he received the Nobel Prize.

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Some incoming sunlight is reflected by the Earth's atmosphere and surface, but most is absorbed by elipsis

Warming of the Earth's surface and lower atmosphere caused by water vapour, carbon dioxide, and other trace gases in the atmosphere. Visible light from the Sun heats the Earth's surface. Part of this energy is radiated back into the atmosphere in the form of infrared radiation, much of which is absorbed by molecules of carbon dioxide and water vapour in the atmosphere and reradiated toward the surface as more heat. (Despite the name, the greenhouse effect is different from the warming in a greenhouse, where panes of glass allow the passage of visible light but hold heat inside the building by trapping warmed air.) The absorption of infrared radiation causes the Earth's surface and lower atmosphere to warm more than they otherwise would, making the Earth's surface habitable. An increase in atmospheric carbon dioxide caused by widespread combustion of fossil fuels may intensify the greenhouse effect and cause long-term climatic changes. Likewise, an increase in atmospheric concentrations of other trace greenhouse gases such as chlorofluorocarbons, nitrous oxide, and methane resulting from human activities may also intensify the greenhouse effect. From the beginning of the Industrial Revolution through the end of the 20th century, the amount of carbon dioxide in the atmosphere increased 30percnt and the amount of methane more than doubled. It is also estimated that the U.S. is responsible for about one-fifth of all human-produced greenhouse-gas emissions. Seealso global warming.

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or domino effect

Doctrine of U.S. foreign policy during the Cold War, according to which the fall of a noncommunist state to communism would precipitate the fall of other neighbouring noncommunist states. The theory was first enunciated by Pres. Harry Truman, who used it to justify sending U.S. military aid to Greece and Turkey in the late 1940s. Dwight D. Eisenhower, John F. Kennedy, and Lyndon B. Johnson invoked it to justify U.S. military involvement in Southeast Asia, especially the prosecution of the Vietnam War.

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Splitting of a spectral line (see spectrum) into two or more lines of different frequencies. The effect occurs when the light source is placed in a magnetic field. It has helped identify the energy levels in atoms; it also provides a means of studying atomic nuclei and electron paramagnetic resonance (see magnetic resonance) and is used in measuring the magnetic field of the Sun and other stars. It was discovered in 1896 by Pieter Zeeman (1865–1943); he shared the second Nobel Prize for Physics (1902) with Hendrik Antoon Lorentz, who had hypothesized that a magnetic field would affect the frequency of the light emitted.

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Development of a transverse electric field in a solid material carrying an electric current and placed in a magnetic field perpendicular to the current. Discovered in 1879 by Edwin H. Hall (1855–1938), the Hall field results from the force exerted by the magnetic field on the moving particles of the current. The Hall effect can be used to measure certain properties of current carriers as well as to detect the presence of a current on a magnetic field.

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Apparent difference between the frequency at which waves—including light, sound, and radio waves—leave a source and that at which they reach an observer. The effect, first described by the Austrian physicist Christian Doppler (1803–1853), is caused by the relative motion of the observer and the wave source. It can be observed by listening to the blowing horn or siren of an approaching vehicle, whose pitch rises as the vehicle approaches the observer and falls as it recedes. It is used in radar and to calculate the speed of stars by observing the change in frequency of their light.

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Change in wavelength of X rays and other energetic forms of electromagnetic radiation when they collide with electrons. It is a principal way in which radiant energy is absorbed by matter, and is caused by the transfer of energy from photons to electrons. When photons collide with electrons that are free or loosely bound in atoms, they transfer some of their energy and momentum to the electrons, which then recoil. New photons of less energy and momentum, and hence longer wavelength, are produced; these scatter at various angles, depending on the amount of energy lost to the recoiling electrons. The effect demonstrates the nature of the photon as a true particle with both energy and momentum. Its discovery in 1922 by Arthur Compton was essential to establishing the wave-particle duality of electromagnetic radiation.

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Effect, from Latin effectus "performance, accomplishment" can be used in various meanings:

Please note that although the word "effect" is most commonly found in noun form, it also exists as a verb, and as such is often confused with the word "affect" (itself most commonly a verb, but occasionally found in noun form, especially in the areas of psychology and philosophy).

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