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Proof of Kirchhoff’s Law

Kirchhoff’s law can be derived in a simplified manner as follows:

Suppose Q is the amount of radiation incident per unit area per second on a body. If As is the absorptive power of the surface, then the amount of radiation absorbed by the body per unit area per second = ASX Q.

If ES represents the emissive power of the surface, then the amount of radiation emitted by the surface per unit area per second = ES.

When the body is in thermal equitbrium with the enclosure,
Amount of radiation emitted by the surface per unit area per second = Amount of radiating absorbed by the per unit area per second.

Thus
                                                                           
For a perfectly black body, As = AB  =1 and Es may be replaced by EB where AB and EB represent the absorptive power and the emissive power of a perfectly black body. Thus we have
                                              
From equation
                                                
or                                       

As EB is constant, the equation gives the expression for the Kirchhoff’s law.

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