On the validity of Kirchhoff's law of thermal emission

Abstract

In this paper, Kirchhoff's law is discussed in the context of two extremes: the perfect absorber and the perfect reflector. It is argued that Kirchhoff's extension of his law to the perfect reflector is not justified based on experimental evidence. This greatly limits the universality of the formulations advanced by Kirchhoff and Planck in that blackbody radiation becomes dependent on the nature of the radiating object. In this regard, it is emphasized that graphite is unique in its ability to act as a nearly perfect absorber. The consequences are important in our analysis of all temperatures based on radiative emission.