In thermodynamics, a black body (English: Black body) is an idealized object that can absorb all external electromagnetic radiation without any reflection or transmission. As the temperature rises, the electromagnetic waves and light radiated by the black body are called black body radiation.
A black body has an absorption coefficient of 1 and a transmission coefficient of 0 for electromagnetic waves of any wavelength. But the black body is not necessarily black, even if it does not reflect any electromagnetic waves, it can also emit electromagnetic waves, and the wavelength and energy of these electromagnetic waves are all determined by the temperature of the black body and not changed by other factors.
For human vision, a blackbody looks black below 700K, because the radiation energy emitted by a blackbody below 700K (426.85°C) is very small and the radiation wavelength is outside the range of visible light.
If the temperature of the black body is higher than the above temperature, the black body will no longer be black, it will start to turn red, and as the temperature rises, orange, yellow, white and other colors will appear respectively, that is, black body The process of absorbing and emitting electromagnetic waves follows the spectrum, and its trajectory is the Planckian locus (or blackbody locus).
Black body radiation is actually thermal radiation from a black body. In the spectrum of a blackbody, since high temperatures cause high frequencies, i.e. short wavelengths, higher temperature blackbodies are closer to the blue region at the end of the spectrum and lower temperature blackbodies are closer to the red region.
At room temperature, the black body basically emits infrared rays, but when the temperature rises above 100 degrees, the black body begins to emit visible light, which changes into red, orange, yellow, white and blue according to the temperature increase process. When the blackbody turns white, it emits a large amount of ultraviolet light at the same time.