Wien's distribution law

A relation, derived on purely thermodynamic reasoning by Wien, between the monochromatic emittance of an ideal blackbody and that body’s temperature.

where e_λ is the monochromatic emittance (emissive power) of a blackbody at wavelength λ and absolute temperature T, and f(λT) a function that cannot be determined purely on classical thermodynamic grounds. The specification of f(λT) by Planck in 1901 formed the beginning of modern quantum theory.
Compare Wien’s law.