It is useful to examine the above results in limits of high and low photon energy. For which represents the classical Boltzmann distribution . Photons act like particles. For , which is in the Rayleigh-Jeans Limit . In a single phase cell, .
Photons act like waves.
An example of a blackbody spectrum is shown in the figure below. The above curve represents a perfect blackbody to within the experimental errors which tells us that the 3 K radiation was originally emitted under conditions of thermodynamic equilibrium. Since the universe today consists of both matter and radiation, these two components were, at one time, in thermodynamic equilibrium. That is the subject of the next section.