Closer observation reveals that heated objects radiate photons over a wide range of wavelengths.
The distribution of wavelengths is peaked and the peak frequency moves up as the object's temperature increases. We'll be able to account for this behavior quantitatively by applying what we've learned about the statistical properties of photon gases. Consider a rectangular cavity of dimensions in equilibrium with a thermal reservoir at temperature . The walls of this container are made of atoms which continuously absorb and emit photons of different wavelengths. Our task is to calculate how many photons of each color are contained within the box at .