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The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as

The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law.

Kirchhoff's law of thermal radiation states that the emissive power to the coefficient of absorption is constant for all the substances at a given temperature. We can say that at a given temperature, the emissivity of a substance is equal to the coefficient of absorption.

This leads to the observation that if an object absorbs 100 percent of the radiation incident upon it, it must reradiate 100 percent.