Stark, Johannes (1874-1957)

German physicist. He was awarded the Nobel Prize for Physics in 1919 for his discovery in 1902 that high-velocity rays of positive ions (canal rays) would demonstrate the Doppler effect, and his demonstration in 1913 that a strong electric field can alter the wavelength of light emitted by atoms (the Stark effect).

Stark modified in 1913 the photo-equivalence law proposed by Albert Einstein in 1906. Now called the Stark-Einstein law, it states that each molecule involved in a photochemical reaction absorbs only one quantum of the radiation that causes the reaction.

Stark was born in Schickenhof, Bavaria, and studied at Munich.

He became professor in 1906 at the Technische Hochschule in Hannover and subsequently held other academic posts until 1922, when he attempted to set up a porcelain factory. This scheme failed, largely because of the depressed state of the German economy. Stark joined the Nazi party in 1930 and three years later became president of the Reich Physical-Technical Institute and also president of the German Research Association. But his attempts to become an important influence in German physics brought him into conflict with the authorities and he was forced to resign in 1939. After World War II, he was sentenced to four years' imprisonment by a denazification court in 1947.