Definition 2: an idealized comparison standard when studying the performance of actual objects to absorb and emit radiation energy.
Features: Any object has the ability to continuously radiate, absorb and emit electromagnetic waves. The radiated electromagnetic wave is different in each band, that is, it has a certain frequency spectrum distribution. This spectral distribution is related to the characteristics of the object itself and its temperature, so it is called thermal radiation. In order to study the law of thermal radiation independent of the specific physical properties of matter, physicists have defined an ideal object-blackbody as the standard object for thermal radiation research.
The so-called blackbody means that all the incident electromagnetic waves are absorbed, neither reflected nor transmitted (of course, blackbody still radiates outwards). Kirchhoff's radiation law, the ratio of energy radiated by an object to absorption rate in thermal equilibrium, has nothing to do with the physical properties of the object itself, but only with wavelength and temperature. According to Kirchhoff's radiation law, at a certain temperature, the blackbody must be the object with the greatest radiation ability. It can be called a complete radiator. Blackbody radiation refers to the radiation emitted by ideal radiation, which emits the largest amount of radiation at a specific temperature and wavelength. At the same time, the blackbody is an object that can absorb all the incident radiation and will not reflect any radiation, so the blackbody is absolutely black. Theoretically, the black body emits electromagnetic waves of all wavelengths in the spectrum. Wien's displacement law is a law describing the relationship between the peak wavelength of blackbody electromagnetic radiation energy flow density and its own temperature.