For a general introduction, see black body.

In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature T. As a function of frequency ν, Planck's law is written as:^[1]

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 u(ν,T) = πI(ν,T) for emitted power integrated over all solid angles. In other cases, it is written as u(ν,T) = 4πI(ν,T) / c for energy per unit volume.

The function I(ν,T) peaks for hν = 2.82kT.^[2] It falls off exponentially at higher frequencies and polynomially at lower.