<P> Thus Kirchhoff's law of thermal radiation can be stated: For any material at all, radiating and absorbing in thermodynamic equilibrium at any given temperature T, for every wavelength λ, the ratio of emissive power to absorptive ratio has one universal value, which is characteristic of a perfect black body, and is an emissive power which we here represent by B (λ, T). (For our notation B (λ, T), Kirchhoff's original notation was simply e .) </P> <P> Kirchhoff announced that the determination of the function B (λ, T) was a problem of the highest importance, though he recognized that there would be experimental difficulties to be overcome . He supposed that like other functions that do not depend on the properties of individual bodies, it would be a simple function . Occasionally by historians that function B (λ, T) has been called "Kirchhoff's (emission, universal) function," though its precise mathematical form would not be known for another forty years, till it was discovered by Planck in 1900 . The theoretical proof for Kirchhoff's universality principle was worked on and debated by various physicists over the same time, and later . Kirchhoff stated later in 1860 that his theoretical proof was better than Balfour Stewart's, and in some respects it was so . Kirchhoff's 1860 paper did not mention the second law of thermodynamics, and of course did not mention the concept of entropy which had not at that time been established . In a more considered account in a book in 1862, Kirchhoff mentioned the connection of his law with Carnot's principle, which is a form of the second law . </P> <P> According to Helge Kragh, "Quantum theory owes its origin to the study of thermal radiation, in particular to the "black - body" radiation that Robert Kirchhoff had first defined in 1859--1860 ." </P>

To understand blackbody radiation physicists should view light as having what type of properties