On This Day
1914 – The Franck–Hertz experiment, a pillar of quantum mechanics, is presented to the German Physical Society.
The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms, and thus “transformed our understanding of the world”.[attribution needed][1] It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz.[2][3] Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away.[4] This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero.[5] A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.
These experimental results proved to be consistent with the Bohr model for atoms that had been proposed the previous year by Niels Bohr. The Bohr model was a precursor of quantum mechanics and of the electron shell model of atoms. Its key feature was that an electron inside an atom occupies one of the atom’s “quantum energy levels”. Before the collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 electron volts (eV) more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities in Bohr’s quantum model. This feature was “revolutionary” because it was inconsistent with the expectation that an electron could be bound to an atom’s nucleus by any amount of energy.[4][6]
In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions.[7] They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr.[4] After a presentation of these results by Franck a few years later, Albert Einstein is said to have remarked, “It’s so lovely it makes you cry.”[1]
On December 10, 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics “for their discovery of the laws governing the impact of an electron upon an atom”.[8]
Born On This Day
1880 – Gideon Sundback, Swedish-American engineer and businessman, developed the zipper (d. 1954)
Gideon Sundback (April 24, 1880 – June 21, 1954) was a Swedish-American electrical engineer, who is most commonly associated with his work in the development of the zipper.[1]
FYI
By Shannon Miller: R.I.P. prolific American playwright Mark Medoff
Mark Medoff (March 18, 1940 – April 23, 2019) was an American playwright, screenwriter, film and theatre director, actor, and professor. His play Children of a Lesser God received both the Tony Award and the Olivier Award. He was nominated for an Academy Award and a Writers Guild of America Best Adapted Screenplay Award for the film script of Children of a Lesser God and for a Cable ACE Award for his HBO Premiere movie, Apology. He also received an Obie Award for his play When You Comin’ Back, Red Ryder?[citation needed] Medoff’s feature film Refuge[1] was released in 2010.
When You Comin’ Back, Red Ryder? was adapted into a film with a screenplay by Medoff in 1979.[2]
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- Wikimedia Commons picture of the day for January 21 January 21, 2025Picture of the day Gonbo Rangjon with peak elevation 5,520 m (18,110 ft) towers 1,200 m (3,900 ft) above the Kargiakh valley in Zanskar, Ladakh, India. The tents in the foreground indicate the Himalayan scale. Known locally as God's Mountain, climbing is not permitted. The snow-capped range to the right includes the Shinko La pass to Lahaul.
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