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black body, in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, approximates an ideal black body. Since a black body is a perfect absorber of radiant energy, by the laws of thermodynamics it must also be a perfect emitter of radiation. The distribution according to wavelength of the radiant energy of a black body radiator depends on the absolute temperature of the black body and not on its internal nature or structure. As the temperature increases, the wavelength at which the energy emitted per second is a maximum decreases. This phenomenon can be seen in the behavior of an ordinary incandescent object, which gives off its maximum radiation at shorter and shorter wavelengths as it becomes hotter and hotter. First it glows in long red wavelengths, then in yellow wavelengths, and finally in short blue wavelengths. In order to explain the spectral distribution of black body radiation, Max Planck developed the quantum theory in 1901. In thermodynamics the principle of the black body is used to determine the nature and amount of the energy emitted by a heated object. Black-body radiation has served as an important source of confirmation for the big-bang theory, which holds that the universe was born in a fiery explosion some 10 to 20 billion years ago. According to the theory, the explosion should have left a remnant black-body cosmic background radiation that is uniform in all directions and has an equivalent temperature of only a few degrees Kelvin. Such a uniform background, with a temperature of 2.7°K (see Kelvin temperature scale), was discovered in 1964 by Arno A. Penzias and Robert L. Wilson, who were awarded the Nobel Prize in Physics in 1978 for their work. Recent data gathered by the NASA satellite Cosmic Microwave Background Explorer (COBE) has revealed small temperature fluctuations in the radiation that are thought to be related to the "seeds" of stars and galaxies.
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