**Stefan’s law** states that the total amount of heat energy radiated per second per unit area of a perfect black body is directly proportional to the fourth power of its absolute temperature.

E α T^{4} or E = σ T^{4}

where σ is called Stefan’s constant.

Its value is 5.67 x 10^{-8} W m^{-2} K^{-4}

It is also called Stefan – Boltzmann law, as Boltzmann gave a theoretical proof of the result given by Stefan.