1.

State and explain Kirchhoff’s law.

Answer»

At a given temperature, the ratio of the emissive power to the absorptive power for all bodies is a constant and is equal to the emissive power of a perfect black body.

If e is the emissive power and a is the absorptive power then, e/a = constant = 1.

∴ e = a (∵ emissive power of a perfectly black body = 1) 

Thus, the emissive power of a body is equal to its absorptive power, which means good emitter of thermal radiation is also a good absorber of heat.



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