The theory of thermal radiation



Max Planck began to take an interest in investigating radiative equilibria in the mid1890s. At that time physicists knew very little about the regularies according to which a heated object, for example, emits thermal and luminous radiation. The properties of thermal radiation can be described with the help of a model, what is called a ``black body''. The radiation emitted from such a body does not depend on the material that it is made of. It obeys a universal function dependent on temperature and frequency. Constructing such a black body was a challenging problem in itself, just as was finding that universal function.
The national institution of standard weights and measures in Germany, the PhysikalischTechnische Reichsanstalt (PTR) in Berlin, was then the center of radiation researchnot least because of its practical implications for the nascent lamp industry. In 1896 Wilhelm Wien, a member of the staff at that institution, was the first to suggest a law that seemed to be empirically supported, describing the spectral distribution of energy as a function of temperature. Three years later Planck managed to derive this semiempirically obtained radiation formula theoretically. But precision measurements taken at the PTR soon revealed that in the range of long waves there were considerable deviations from the radiation formula by Wien and Planck. New measurements agreed with a radiation formula that the English physicist Lord Rayleigh had just recently published.
The lack of conformity with the measurement data prompted Planck to reconsider the problem. In October 1900 he proposed a formula that agreed with the new empirical data. But his "lucky guess for the interpretational formula" still needed theoretical explanation. After weeks of concentrated work he was able to provide this as wellon 14 December 1900 he presented his report at a meeting of the Physikalische Gesellschaft in Berlin. For his theoretical derivation of the radiation law Planck had had to revise his earlier physical assumptions completelyamong other things he had to assume that the energy e is not a continuous quantity but a discrete one, proportional to its frequency n (e = hn). The introduction of a natural constant h, the Planck quantum of action, was a radically bold assumption, since it contradicted the basic assumption of classical physics that nature did not make any leaps. 
"What always interested me primarily in physics were the great general laws that are of significance in all natural processes, irrespective of the properties of the bodies undergoing the processes.'' Max Planck, 1943 