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 states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.
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