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|See also: Planck's Radiation Law|
At a higher temperature, all objects will send a continuous spectrum of waves, which reaches far beyond the long-wave end of the visible range. A portion of this radiation is noticed on our skin with a temperature sensation. Already near a conventional oven, one can sense a strong, warm feeling due to the radiation which the oven radiates. At a still higher temperature, the furnace starts glowing, even emitting visible light. The extent and intensity of the radiation depend on the temperature of the object. Since the movement of the atoms and molecules theoretically only stops at absolute zero, cold objects also give off radiation.
Due to emission, objects warmer than their environment will gradually lose their heat energy. When temperatures are the same then a radiation equilibrium will exist, i.e. for each unit of time, the object receives the same amount of radiation energy from the environment that it gives up.
When radiation meets matter, there are familiar phenomena related to transmission, reflection and absorption which occur. In the latter case, the radiation is more or less absorbed and in this way contributes to the increase in its temperature. It is well-known that dark objects feature strong absorption. Black paper absorbs 95%, white only 5% of light that strikes it. Matter that completely absorbs all incidental radiation is called a black body. Since even soot still reflects about 1%, a black body can only be realized if a black cavity has a small opening so that reflection can be kept arbitrarily small. Thus, examples of black matter would be the blasting hole of a furnace or at a greater distance, the window of a house.
In physics a black body is an object that absorbs all light that falls onto it: no light passes through it nor is reflected. Despite the name, black bodies do radiate light. The term "black body" was introduced by Gustav Kirchhoff in 1862. The light emitted by a black body is called black-body radiation.
Even the condition that the nature of the matter is not significant reflects the general concept of calculating the black body. The measurements of radiation intensity and their dependence on temperature and frequency were mainly carried out by Lummer, Pringsheim, Rubens and Kurlbaum about 1900. What was missing was a formula that would correspond to measurements for all frequencies and temperatures. Only Max Planck was able to solve these contradictions (Planck's Radiation Law).
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