According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a

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.