- Person
- James Franck
- Lived
- 1882–1964
- Field
- Atomic physics
- From
- German-American
The story
In 1913 Niels Bohr proposed that electrons in atoms can only occupy certain fixed energy levels. It was a bold idea, and it needed direct experimental proof.
James Franck, the son of a Jewish banking family from Hamburg, was then a young lecturer at the University of Berlin. His most fruitful collaboration was with Gustav Hertz, with whom he wrote 19 papers. In 1914 the two men built a vacuum tube containing a drop of mercury, heated it to about 115 °C to fill it with a thin vapour, and fired electrons through it towards a metal grid. Slow electrons simply bounced off the mercury atoms. But once an electron had enough energy, it would lose exactly 4.9 electronvolts in a collision, never more and never less. They presented these results to the German Physical Society on 24 April 1914.
In a second paper that May, they showed that the mercury then glowed with ultraviolet light, at a wavelength of about 254 nanometres, which corresponded precisely to that amount of energy. This was clear evidence that atoms absorb energy only in discrete packets, as Bohr’s model required. At first Franck and Hertz did not realise the connection; in his Nobel lecture Franck admitted they had not even mentioned Bohr’s theory. Their final joint paper, in December 1918, reconciled their results with it. After Franck later presented the findings, Albert Einstein is said to have remarked that it was so lovely it made you cry.
Franck and Hertz received the 1925 Nobel Prize in Physics, presented on 10 December 1926, for their discovery of the laws governing the impact of an electron upon an atom. It was the first electrical measurement to show clearly the quantum nature of atoms.
Their experiment is still performed by physics students around the world as the classic demonstration of quantum energy levels, using teaching tubes built on the same plan. Franck’s later work on how excited atoms hold and pass on energy also produced results that proved important in the development of the laser.
Why it matters
The experiment helped establish quantum theory, the physics behind lasers, LEDs, fluorescent lamps and modern electronics.
About James Franck
Nobel physicist whose electron-collision experiment proved atoms have quantised energy levels; he resigned in protest in 1933 and later led the Franck Report against a surprise atomic attack.
More about James Franck
- James Franck championed women in physicsJames Franck actively advanced the careers of women physicists, notably Hertha Sponer and Hilde Levi, at a time when few women held research posts.
- The Franck–Condon principle explains how molecules glowIn 1925 James Franck proposed the mechanism, later developed by Edward Condon into the Franck–Condon principle, that governs how molecules absorb and emit light.
- James Franck made Göttingen a world centre of physicsFrom 1920 to 1933 James Franck and Max Born made Göttingen a world centre of quantum physics, training scientists such as Blackett, Condon and Oppenheimer.
- James Franck devoted his later years to photosynthesisFrom the 1930s James Franck studied how plants turn light into chemical energy, heading Chicago's Photosynthesis Research Group until 1956.
- James Franck resigned in protest against Nazi lawsOn 17 April 1933 James Franck, exempt as a war veteran, resigned his Göttingen chair in protest, the first German academic to do so over the Nazi civil service law.
- The Franck Report urged a warning before the bombOn 11 June 1945 the Franck Report urged that the atomic bomb be demonstrated in an uninhabited place rather than used on Japanese cities without warning.
- Franck helped dismissed Jewish scientists find workBefore leaving Germany in November 1933, James Franck helped Frederick Lindemann of Oxford find posts abroad for Jewish scientists dismissed by the Nazis.
Sources
- James Franck Wikipedia
- James Franck – Biographical NobelPrize.org
- Franck–Hertz experiment Wikipedia
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.