So, I have some problems understanding Kirchoff's Radiation law.
My textbook, Transport Processes and Separation Process Principles, by Geankoplis, states that at the same temperature T1 the emissivity and absorptivity of a surface is equal, which holds for any black or non black solid surface.
In a problem from my professor it is given that : The sun radiate a flat surface with 1000 W/m2. The absorptivity of the plate is 0.9 and the emissivity is 0.1. The air temperature is 20 C and the heat transfer coefficient is 15 W/Km2. Calculate the surface temperature at equilibrium if the bottom is isolated.
My question is: how is it possible that the emissivity and absorptivity in this case is not equal, which contradicts Kirchoff's law?