The absolutely black body is called such because it absorbs all the radiation falling on it (or rather, into it) both in the visible spectrum and beyond. But if the body does not heat up, the energy is re-radiated back. This radiation emitted by a completely black body is of particular interest. The first attempts to study its properties were made even before the appearance of the model itself.
In the early 19th century, John Leslie experimented with various substances. As it turned out, black soot not only absorbs all the visible light falling on it. It radiated in the infrared range much stronger than other, lighter, substances. It was thermal radiation, which differs from all other types in several properties. The radiation of an absolutely black body is equilibrium, homogeneous, occurs without energy transfer and depends only on the temperature of the body.
When the temperature of the object is high enough, thermal radiation becomes visible, and then any body, including absolutely black, acquires color.
Such a unique object that emits only a certain type of energy, could not help but attract attention. Since we are talking about thermal radiation, the first formulas and theories about what the spectrum should look like were proposed within the framework of thermodynamics. Classical thermodynamics was able to determine at what wavelength the maximum radiation should be at a given temperature, in which direction and how much it will shift when heated and cooled. However, it was not possible to predict what is the distribution of energy in the black body spectrum at all wavelengths and, in particular, in the ultraviolet range.
According to classical thermodynamics, energy can be emitted in any portions, including arbitrarily small ones. But in order for an absolutely black body to radiate at short wavelengths, the energy of some of its particles must be very large, and in the region of ultrashort waves it would go to infinity. In reality, this is impossible, infinity appeared in the equations and was called the ultraviolet catastrophe. Only Planck's theory that energy can be radiated in discrete portions - quanta - helped to resolve the difficulty. Today's equations of thermodynamics are special cases of the equations of quantum physics.
Initially, a completely black body was represented as a cavity with a narrow opening. Radiation from outside enters such a cavity and is absorbed by the walls. On the spectrum of radiation, whichmust have an absolutely black body, in which case the spectrum of radiation from the entrance to the cave, the opening of the well, the window to the dark room on a sunny day, etc. is similar. But most of all, the spectra of the relic radiation of the Universe and stars, including the Sun, coincide with it.
It is safe to say that the more particles with different energies in an object, the stronger its radiation will resemble a black body. The energy distribution curve in the spectrum of a black body reflects the statistical patterns in the system of these particles, with the only correction that the energy transferred during interactions is discrete.
