In physics, the Rayleigh-Jeans Law, first proposed in the 19th century, expresses the energy density of blackbody radiation of wavelength λ as

where T is the temperature in kelvins, and k is Boltzmann's constant.

The law is derived from classical physics arguments. It agrees with experimental measurements for long wavelengths. However it disagrees with experiment at short wavelengths, where it diverges and predicts an unphysical infinite energy density. This failure is known as the ultraviolet catastrophe.

Max Planck revised the law as follows:

where h is Planck's constant and c is the speed of light. This is Planck's law of black body radiation expressed in terms of wavelength λ = c/ν. The Planck law does not suffer from an ultraviolet catastrophe. In the limit of very high temperatures or long wavelengths, the term in the exponential becomes small, and so the denominator becomes approximately hc/λkT. This gives back the Rayleigh-Jeans Law.

See also

• John William Strutt, 3rd Baron Rayleigh