The Solvay Councils and the Birth of Modern Physics by Pierre Marage & Grégoire Wallenborn

Author:Pierre Marage & Grégoire Wallenborn
Language: eng
Format: epub
Publisher: Springer Basel AG 1999
Published: 2015-04-07T16:00:00+00:00

The photoelectric effect

Hertz had already observed that the appearance of sparks was enhanced when his electromagnetic wave detector was illuminated by ultraviolet light. Study of this “Hertz effect” had verified, as one might expect, that the number of extracted electrons increased with light intensity, but it was also observed that their speed (and hence their kinetic energy) remained constant. However, according to classical theory, the energy of the extracted electrons should have increased with the wave intensity (since the energy of the waves is proportional to intensity). On the other hand, for the speed of the electrons to increase, the wave frequency also had to increase. These effects can be explained if the energy of the electromagnetic wave is itself transported as quanta: the increase in intensity corresponds to an increase in the number of quanta, but their energy is constant, and each liberated electron has a fixed speed.

These are the observations which Planck mentioned in his report in 1911: “As an important confirmation of this hypothesis (of Einstein), we can cite the fact that the speed of secondary cathode rays produced by the rays of RÖntgen (i.e. X-rays) is independent of their intensity. J. J. Thomson also arrived at a similar idea through studying photoelectric phenomena. He found that it was only possible to explain the small number of emitted electrons and the fact that their speed is unrelated to the intensity of the incident light, by allowing localised packets of energy instead of a uniform partition of energy in the light wave front.

The reports of the proceedings from the first Solvay Physics Council, published in 1912.

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