Quantum Nature of Atoms: “The Franck-Hertz Experiment” 1961 PSSC; Byron Youtz; James Franck

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“Professor Byron L Youtz [Reed College] uses a stream of electrons accelerated through mercury vapor to show that the kinetic energy of the electrons is transferred to the mercury atoms only in discrete packets of energy. Establishes the association of the quantum of energy with a line in the spectrum of mercury. Points out that the Frank-Hertz experiment was one of the earliest indications of the existence of internal energy states within the atom. Includes an epilogue by James Franck.”

Originally a public domain film, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

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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”. It was presented on April 24, 1914 to the German Physical Society in a paper by James Franck and Gustav Hertz. 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. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. 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.

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. 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. 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.”

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.”…

The Physical Science Study Committee, usually abbreviated as PSSC, was inaugurated at a 1956 conference at MIT to review introductory physics education and to design, implement, and monitor improvements. It produced major new physics textbooks, instructional movies, and classroom laboratory materials, which were used by high schools around the world during the 1960s and 1970s and beyond…

By the 1964–1965 school year, about half the US students enrolled in high school physics (200,000 students, 5000 teachers) were reportedly using the PSSC course materials…