- ν is the frequency
*I*(ν)δν is the amount of energy per unit surface per unit time per unit solid angle emitted in the frequency range between ν and ν+δν;*h*is Planck's constant,:*c*is the speed of light and*k*is Boltzmann's constant.

The Rayleigh-Jeans law was particularly significant, since it was built on a strong theoretical framework, but suffered a serious flaw known as the ultraviolet catastrophe. This suggested that the theoretical framework of thermodynamics was faulty. Planck now attempted to produce a better fundamental theory which would supplement thermodynamics. He noted that if light could only be emitted in discrete packets with energy proportional to frequency, the new law would make perfect sense.

*E*=*h*ν

From the Planck's law of black body radiation we derive today the Stefan-Boltzmann law.

- Planck, Max, "
*On the Law of Distribution of Energy in the Normal Spectrum*". Annalen der Physik, vol. 4, p. 553 ff (1901).