This occurs regardless of the wavelength of the radiation entering (as long as it is small compared to the hole). 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. This makes peepholes into furnaces good sources of blackbody radiation, and some people call it cavity radiation for this reason. Conversely, the hole acts as a nearly ideal black-body radiator. If a small window is opened into an oven, any light that enters the window has a very low probability of leaving without being absorbed. At room temperature, black bodies emit infrared light, but as the temperature increases past a few hundred degrees Celsius, black bodies start to emit at visible wavelengths, from red through orange, yellow, and white before ending up at blue, beyond which the emission includes increasing amounts of ultraviolet radiation. The temperature of the object is directly related to the wavelengths of the light it emits. If a perfect black body at a certain temperature is surrounded by other objects in equilibrium at the same temperature, it will on average emit exactly as much as it absorbs, at the same wavelengths and intensities of radiation that it had absorbed. When heated, the black body becomes an ideal source of thermal radiation, which is called black-body radiation. Because it does not reflect or transmit visible light, the object appears black when it is cold. In physics, a black body (in an ideal sense) is an object that absorbs all electromagnetic radiation that falls on it, without any of the radiation passing through it or being reflected by it.
The black-body radiation graph is also compared with the classical model of Rayleigh and Jeans. As the temperature decreases, the peak of the black-body radiation curve moves to lower intensities and longer wavelengths.
Black-body radiation curves at different temperatures: 3000 K, 4000 K, and 5000 K.
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