Answer to Question #230379 in Molecular Physics | Thermodynamics for Ebrahim

Question

How will you
verify experimentally that the ratio of the emissive and absorptive power is the same for all bodies
and is equal to the emissive power of a perfectly black body?

Answer 1

Answer: Kirchhoff's law: the ratio of the radiative capacity of the body to its absorptive capacity does not depend on the material of the body; it is for all bodies a universal function of frequency and temperature T, this function is the radiative capacity of an absolutely black body.

This can be seen by considering a heat-insulated system consisting of two infinitely long plates AA1 and BB1, which can exchange energy in the form of heat only with each other, because their outer surfaces are covered with ideal thermal insulation. Let the inner surface of the plate AA1 be absolutely black, and the emissivity and absorption capacity of the inner surface of the plate BB1 ​​be equal to $r_{\nu,T}$ and $\alpha_{\nu,T}$ . If thermodynamic equilibrium is established in this system, then the temperatures of both plates are the same and equal to T, and the radiation of the plates is equilibrium.