is the frequency of the radiation. Indeed, it is possible to derive Planck's law of radiation from this assumption alone, as shown by Debye, who employed a method which is a combination of that of Planck and of Jeans. Before considering any further the nature of the oscillating systems let us see whether it is possible to bring this assumption about the emission of radiation into agreement with the spectral laws.
If the spectrum of some element contains a spectral line corresponding to the frequency
it will be assumed that one of the atoms of the element (or some other elementary system) can emit an amount of energy
. Denoting the energy of the atom before and after the emission of the radiation by
we have
