Thus, if we take the classical limit, we recover ex-
actly the expression for the average energy of a har-
monic oscillator in thermal equilibrium, E = kT.
We can now complete the determination of Planck's
radiation formula. We have already shown that
the number of modes in the frequency interval º to
º+dº is (8¼º2=c3) dº per unit volume. The energy
The Planck distribution in terms of
the energy density of radiation per
unit frequency interval
density of radiation in this frequency range is