Black body

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A black body is an idealized physical body that absorbs all electromagnetic radiation falling on it. Because of its perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body's temperature. At Earth-ambient, low temperatures this emission is in the infrared region of the electromagnetic spectrum and not visible, and therefore the object appears black, since it does not reflect or emit any visible light.

The thermal radiation from a black body is energy converted electrodynamically from the body's pool of internal thermal energy at any temperature greater than absolute zero. It is called blackbody radiation and has a distribution with a frequency maximum that shifts to higher energies with increasing temperature. As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, appearing red, orange, yellow, white, and blue with increasing temperature. By the time an object is visually white, it is emitting a substantial fraction as ultraviolet light.

Blackbody emission provides insight into the thermodynamic equilibrium state of the source of a continuous field. According to the equipartition theorem in classical physics, each Fourier mode or degree of freedom should have the same energy when in equilibrium. This approach led to the paradox known as the ultraviolet catastrophe, that there would be an infinite amount of energy in any continuous field. The study of the laws of the black body led to some of the foundations of quantum mechanics.

The term black body was introduced by Gustav Kirchhoff in 1860. When used as a compound adjective, the term is typically written as one word in US English, e.g. in blackbody radiation, but sometimes also hyphenated, as in black-body radiation.