In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature . As a function of frequency , Planck's law is written as: This function represents the emitted power per unit area of emitting surface, per unit solid angle, and per unit frequency. Sometimes, Planck's law is written as an expression for emitted power integrated over all solid angles. In other cases, it is written as for energy per unit volume. The function peaks for h = 2.82kT. It falls off exponentially at higher frequencies and polynomially at lower.