Define Kirchhoff's law of radiation.

Kirchhoff's Law refers to the fact that all bodies own the
quality of emission and absorption. At a given temperature (T), in conditions of
thermodynamic equilibrium, the ratio of capacity of energy  and capacity of absorption
of a certain body is a constant value, equal to the capacity of emission of the absolute
black body, both, for radiation with a specific wavelength and for the total radiation
with different wavelengths.

Kirchhoff's law leads to the
following conclusions: a body that is a good absorber is also a good
radiant.

The perfect black body absorbs all of the amount
of radiant energy that falls on its surface. His capacity of absorption is equal to
1.