Stefan’s law
 
Prior to Planck’s revolutionary explanation of the black body curve, two other important laws of radiation had already been discovered. The first, known as Stefan’s law or the Stefan–Boltzmann law, states that the total energy, E_btot , radiated in all wavelengths by a black body per unit area per unit time is proportional to the fourth power of the absolute temperature, T , of the body. Hence,
E_btot= σ T ⁴ Wm⁻² (1)
where σ is known as Stefan’s constant. It appears that Stefan made a lucky guess at the law in 1879; it was deduced theoretically by Boltzmann in 1884. The value of σ may be evaluated by integration of the black body curve and is given by