Stefan, Josef (1835–1893)

Austrian physicist who established one of the basic laws of heat radiation in 1879, since known as the Stefan–Boltzmann law. This states that the heat radiated by a hot body is proportional to the fourth power of its absolute temperature.

Stefan was born in Klagenfurt and studied at Vienna, becoming professor there 1863 and director of the Institute for Experimental Physics 1866.

Stefan deduced his radiation law from experiments done by Irish physicist John Tyndall with a platinum wire. From his law, Stefan was able to make the first accurate determination of the surface temperature of the Sun, obtaining a value of approximately 6,000°C/11,000°F.

In 1884 Ludwig Boltzmann, a former student of Stefan's, gave a theoretical explanation of Stefan's law based on thermodynamic principles and kinetic theory. Boltzmann pointed out that it held only for perfect black bodies, and Stefan had been able to derive the law because platinum approximates to a black body.