4.0 Kirchhoff's Law

Kirchhoff's law states that the ratio of emissive power to absorptive power is same for all bodies at a given temperature and is equal to emissive power of the black body at that temperature. $$\begin{equation} \begin{aligned} \frac{{{E_1}}}{{{a_1}}} = \frac{{{E_2}}}{{{a_2}}} = {\left( {\frac{E}{a}} \right)_{perfect\,black\,body}} \\ \\ \because\;{\left( a \right)_{black\,body}} = 1,{\left( E \right)_{black\,body}} = E\;\left( {say} \right) \\ \\ then,\quad {\left( {\frac{E}{a}} \right)_{for\,any\,surface}} = constant = E \\\end{aligned} \end{equation} $$ and for any type wave lengths they are equal.

From the above equation, $$E \propto a$$So, it is proved that, Good absorbers are also good emitter's.