Absolutely black body and its radiation

The absolutely black body is called so because it absorbs all the radiation incident on it (or rather, into it), both in the visible spectrum and outside it. But if the body does not heat up, the energy reradies back. This radiation, emitted by an absolutely black body, is of particular interest. The first attempts to study its properties were done before the appearance of the model itself.

In the early 19th century John Leslie conducted experiments with various substances. As it turned out, black soot not only absorbs all visible light falling on it. It emitted in the infrared range is much stronger than other, lighter substances. It was thermal radiation, which differs from all other species in several properties. The radiation of an absolutely black body is equilibrium, homogeneous, occurs without energy transfer and depends only on the body temperature. At a sufficiently high temperature of the object, the thermal radiation becomes visible, and then any body, including absolutely black, acquires color.

Such a unique object, which emits only a certain kind of energy, could not help attracting attention. Since we are talking about thermal radiation, the first formulas and theories as to how the spectrum should look have been proposed in the framework of thermodynamics. Classical thermodynamics was able to determine at what wavelength the radiation maximum should be at a given temperature, in which direction and how much it will shift with heating and cooling. However, it was not possible to predict what is the energy distribution in the spectrum of an absolutely black body at all wavelengths and, in particular, in the ultraviolet range.

According to the ideas of classical thermodynamics, energy can be radiated by any portions, including arbitrarily small ones. But for an absolutely black body to emit 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 by discrete portions-quanta-has helped to resolve the difficulty. Today's equations of thermodynamics are particular cases of the equations of quantum physics.

Originally a completely black body was represented as a cavity with a narrow hole. Radiation from the outside enters this cavity and is absorbed by the walls. On the spectrum of radiation, which must have an absolutely black body, then the spectrum of radiation from the entrance to the cave, the holes of the well, the windows into 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 can be stated with certainty that the more particles in a given object possessing different energies, the stronger its radiation will resemble the black-body radiation. The energy distribution curve in the spectrum of an absolutely black body reflects statistical regularities in the system of these particles, with the only correction that the energy transferred during interactions is discrete.