**Trying to write the correct formula of thermal radiation…**

At the end of the XIX century, the dependence of the thermal-radiation energy upon body’s temperature was being described by the Rayleigh-Jeans formula that, however, was not consistent with experiments for the blue light and led to the “ultraviolet catastrophe” — an infinitely high energy of radiation — for shorter wavelengths of light. While working on this problem and based on the previous achievements of Ludwig Boltzmann, Max Planck has obtained a formula that describes the radiation correctly. It contains a constant coefficient that determines a minimal energy (a quantum) of light of a given wavelength, which has been later named after the physicist. At that time, a lot of attention has been paid to radiation of heated bodies in physics. The corresponding basic researches were supported by the practical necessity due to the proliferation of light bulbs.