According to it the radiant energy emitted by a perfectly black body per unit area per sec (i.e., emissive power of black body) is directly proportional to the fourth power of its absolute temperature,
i.e., E ∝ T4 → E = σT4
Where is a constant called Stefan’s constant having dimension [MT-3θ=4] and value 5.67 x 10-8 W/m2 K4.
(i) For ordinary body : e = εE = εσT4
(ii) Radiant energy : If Q is the total energy radiated by the ordinary body then e = Q/Axt = A εσT4
(iii) Radiant power (P) : It is defned as energy radiated per unit time i.e., P = Q/t = A εσT4
(iv) If an ordinary body at temperature T is surrounded by a body at temperature T0, then Stefan’s law may be put as
e = ε σ (T4 - T40)