Sept. 23, 2019

Kirchhoff’s Law:

Kirchhoff’s Law

According to this law the ratio of emissive power to absorptive power is same for all surfaces at the same temperature and is equal to the emissive power of a perfectly balck body at that temperature. 

Hence e1/a1 = e2/a2 = ….. (E/A) Perfectly balck bdoy

A = 1, i.e., e/a = E

    ( eλ/aλ)= (Eλ)black

Now since (eλ)black  is constant at a given temperature, according to this law if a surface is a good absorber of a particular wavelength it is also a good emitter of that wavelength.

This in turn implies that a good absorber is a good emitter (or radiator)