State and explain the Stefan-Boltzmann law.

(N/A) Radiation energy can be transmitted over large distances in the absence of a medium.
The total electromagnetic energy emitted by a substance at absolute temperature $T$ depends on its surface area,its emissivity,and its temperature.
The energy emitted per unit time $(H)$ from a perfect black body is given by:
$H = A \sigma T^4$ (For a perfect black body)
where $A$ is the surface area and $T$ is the absolute temperature.
This relation was proved experimentally by the scientist Stefan in $1879$ and theoretically by Boltzmann in $1884$. Hence,it is called the Stefan-Boltzmann law.
$\sigma$ is known as the Stefan-Boltzmann constant. Its $SI$ unit value is $5.67 \times 10^{-8} \text{ W m}^{-2} \text{ K}^{-4}$ and its dimensional formula is $[M^1 L^0 T^{-3} K^{-4}]$.
Emissivity $(e)$: The ratio of the total emissive power of a surface to the total emissive power of the surface of a perfect black body,kept under the same conditions,is called the 'emissivity' $(e)$ of that surface.
$e = \frac{\text{Total emissive power}}{\text{Emissive power of perfect black body}}$. For a perfect black body,$e = 1$.
Absorptivity $(a)$: The ratio of the radiant energy absorbed by a surface to the total radiant energy incident on the surface is called 'absorptivity' $(a)$.
$a = \frac{\text{Radiant energy absorbed}}{\text{Radiant energy incident}}$. For a perfect black body,$a = 1$.
From the Stefan-Boltzmann law,for any body,we can write:
$H = A e \sigma T^4$ ... $(1)$
If a substance at temperature $T$ is kept in surroundings at temperature $T_S$ (where $T > T_S$),the net rate of heat radiation is:
$H = e \sigma A (T^4 - T_S^4)$ ... $(2)$
For a perfect black body,$e = 1$,so $H = \sigma A (T^4 - T_S^4)$.