Chemical Engineering - Heat Transfer - Discussion

For a body of any arbitrary material emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature. That universal function describes the perfect black-body emissive power. So option C.

Kirchhoff law applies to total radiation and monochromatic radiation but is dependent only on temperature.

According to Kirchhoff's law of thermal radiation, the ratio of a body's total radiating power (emissive power) to its absorptivity depends on both the temperature and the wavelength of the radiation, not just one or the other.

This ratio, which is equivalent to the emissivity of the body, is a universal function that is the same for all bodies at thermal equilibrium, and this function is determined by wavelength and temperature.