As hour of birth will be the day on the 14. December 1900 considered. On this day Max Planck in Berlin has proposed to explain the cavity radiation with an entire new assumption that the “energies of electrons” Ei are correlated with a constant h and they send out light with the frequencies, νij, according to Eij = Ei -Ej = h∙νij. Planck was unlucky because he could not nearly explain his “quantum hypothesis”. A new name was also born: h is called the quantum of action. Einstein followed Planck’s idea 1905 and he declared the light quantum hypothesis.
Both great scientists came not on the idea that the Planck’s constant h could be a Lagrange multiplier as a consequence of the subsidiary condition that the electric charges of the involved particles are conserved. As the boundary condition is to be taken that the radiation of light is in equilibrium with the emitting particles.
Not the energy of light is quantized, but only the frequencies of the radiation in the cavity radiation. The Maxwell equation shows how oscillating electric charge density produce electromagnetic radiation with discrete frequencies. The energies of electrons are also not quantized.