Rayleigh Jeans law Rayleigh and Jeans obtained the expression of energy distribution with frequency in black body radiation. They assumed that the radiation of wavelength l forms standing waves inside the cavity. They calculated the number of modes of vibration within the cavity. Each mode of vibration originates in an oscillator in the cavity wall. Therefore each mode should have two degrees of freedom*, one for kinetic and the other for potential energy. Principle of equipartition of energy gives the energy per degree of freedom, which is (1/2)kT (k is the Boltzmann constant). Therefore energy per mode is kT. Multiplying this with the total number of mode gives the energy contained in the black body radiation per unit volume.

En dn =8 p k T l ^-4 dl (r6)

The law fits well for long wavelengths. For short wavelengths it gives infinite value, which is an absurd result. This is called ‘ultraviolet catastrophe’. It indicated that classical theory cannot account for the black body energy distribution.