Frank Hertz Experiment

James Franck & Gustav Hertz

By: Christopher Kellner

James Frank and Gustav Hertz received the Nobel Prize in 1925 for the Frank-Hertz experiment done in 1914. This experiment helped confirm the Bohr model of the atom by discovering the laws which govern how an electron impacts an atom.

Atomic physics was a new science created in 1913 by Niels Bohr. He did this by making several new hypotheses to explain several discrepancies of glowing bodies and the radiation they emit that could not be explained by classical physics. Several decades before then, it was noted in experiments in the field of spectroscopy that observing glowing gas through a spectrometer revealed many different lines called spectral lines. The relationship between the lines was described by several scientists, the most famous of which was Rydberg. However, the fundamental question of how a single element can emit so many lines could not be answered. Niels Bohr answered this with the following hypotheses:

Each atom can exist in an unlimited number of different states, the so-called stationary states. Each of these stationary states is characterized by a given energy level. The difference between two such energy levels, divided by Planck’s constant h, is the oscillation frequency of a spectral line that can be emitted by the atom.

In addition to these general hypotheses, Bohr also gave specific ones to use with Hydrogen and Helium that let people calculate the spectral lines that matched with experimental data very well.

Unfortunately, the general hypotheses Bohr discovered could not become an experimental fact until an experiment could confirm atoms had different states. That was where Frank and Hertz came in. The Frank-Hertz experiment showed that atoms ...

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...Frank and Hertz thought that the ultraviolet light that was observed was due to this ionization. Neil Bohr published a paper in 1915 that said the findings of the Frank-Hertz experiment better corresponded to his theory of quantum exited states. By 1919, Frank and Hertz mostly accepted that their experiment confirmed Bohr’s quantum model. Frank had this to say.

It is funny how many accidents turn into Nobel Prizes.

You can see this experiment going on in many different places, except they tend to use different atoms. The most common would be neon signs. The main color emitted when a free electron hits a neon atom is red, and with the high voltage, there will be many collisions that produce red light. Also since the voltage is variable, the origin of the multiple red lights shifts very quickly making it seem as if the whole thing is constantly emitting red light.