Black-Body Radiation

Hot objects give off light and heat in the form of electromagnetic radiation whose character changes with temperature. Black-body radiation is such electromagnetic radiation in equilibrium with its material surroundings. By the late 1800s, it was a lively research topic for both theoretical and experimental physicists. Samuel Pierpont Langley (1834–1906) in the United States, and a group of experimental physicists in Germany centered around the Physikalisch-Technische Reichsanstalt (PTR) in Charlottenburg, had developed sophisticated techniques for studying this radiation. Part of their motivation was practical — establishing better absolute temperature scales, and measuring light intensities, at high temperatures (► Black Body).

In December 1900 and January 1901, the German physicist Max Planck (1858– 1947) published three short papers in which he derived a new equation to describe black-body radiation—one that ever since has given excellent agreement with observation. This derivation was the culmination of research Planck had begun in the mid-1890s. In a series of lengthy papers, Planck had combined thermodynamics, in which he was an acknowledged authority, with the new electromagnetic theory of James Clerk Maxwell (1831–1879). He considered the electromagnetic field in equilibrium with what he called “resonators” — electric dipoles oscillating in simple harmonic motion — which represented the material cavity containing the field. By late 1899, he had found a new and more rigorous derivation of Wien's law, an equation describing black-body radiation discovered in 1896 by his friend and colleague Wilhelm Wien (1864–1928), and seemingly in good agreement with experiment.