The hole, then, is a close approximation of a theoretical black body and, if the cavity is heated, the spectrum of the hole's radiation (i.e., the amount of light emitted from the hole at each wavelength) will be continuous, and will not depend on the material in the cavity (compare with emission spectrum). This occurs regardless of the wavelength of the radiation entering (as long as it is small compared to the hole). (this technique leads to the alternative term cavity radiation) Any light entering the hole would have to reflect off the walls of the cavity multiple times before it escaped, in which process it is nearly certain to be absorbed. In the laboratory, black-body radiation is approximated by the radiation from a small hole entrance to a large cavity, a hohlraum. Studying the laws of the black body historically led to quantum mechanics. Black bodies could test the properties of thermal equilibrium because they emit radiation which is distributed thermally. In classical physics, each different Fourier mode in thermal equilibrium should have the same energy, leading to the theory of ultraviolet catastrophe that there would be an infinite amount of energy in any continuous field. The term "black body" was introduced by Gustav Kirchhoff in 1860.īlack-body emission gives insight into the thermal equilibrium state of a continuous field. Īt room temperature, black bodies emit mostly infrared light, but as the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, from red, through orange, yellow, and white before ending up at blue, beyond which the emission includes increasing amounts of ultraviolet. This thermal radiation from a black body is termed black-body radiation.
However, a black body emits a temperature-dependent spectrum of light. Because no light (visible electromagnetic radiation) is reflected or transmitted, the object appears black when it is cold. No electromagnetic radiation passes through it and none is reflected. In physics, a black body is an idealized object that absorbs all electromagnetic radiation that falls on it. The color ( chromaticity) of black-body radiation depends on the temperature of the black body the locus of such colors, shown here in CIE 1931 x,y space, is known as the Planckian locus.
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