The Bell System Technical Journal 



April, 1929 



Electrons and Quanta ^ 



By C. J. DAVISSON 



The experiments by the author and L. H. (iermer, by G. P. Thomson 

 and by others from which the wave properties of electrons are adduced are 

 briefly described. The agreement between the results of these experiments 

 and the prediction of L. de Broglie is pointed out. The wave and corpus- 

 cular properties of electrons are compared with the similar properties of light 

 quanta. 



WHEN I discovered on looking over the announcement of this 

 meeting that Professor Compton is to speak on "X-rays as a 

 Branch of Optics," I reaHzed that I had not made the most of my 

 opportunities. I should have made a similar appeal to the attention 

 of the Society by choosing as my subject, "Electrons as a Branch of 

 Optics." And a ver>' good case can be made out that electrons should 

 be so regarded. During the last few years we have come to recognize 

 that there are circumstances in which it is convenient, if not indeed 

 necessary, to regard electrons as waves rather than as particles, and 

 we are making more and more frequent use of such terms as diffraction, 

 reflection, refraction and dispersion in describing their behavior. If 

 this in itself is not enough to mark electrons as a branch of optics, it 

 is sufficient at least to establish a certain community of ideas between 

 the subjects of optics and electronics which cannot but be of interest 

 to the members of this Society. 



The evidence that electrons are waves is similar to the evidence 

 that light and X-rays are waves. A beam of electrons is scattered by 

 a grating — either the lattice grating of a crystal or an ordinary optical 

 grating — and the intensity of scattering, as measured by the current 

 density of electrons proceeding in different directions, is such as can 

 be explained by assuming as is done in optics that what we are dealing 

 with is the superposition of trains of scattered waves proceeding from 

 the grating elements. In other words, current density of scattered 

 electrons displays in these experiments the same type of spacial 

 distribution as flux density in the analogous experiments in optics, and 

 the observations are given a similar interpretation — an interpretation, 

 that is, in terms of the interference of coherent wave-trains. The 



1 Presented at the Michelson Meeting of the Optical Society, Washington, D. C, 

 November 1-3, 1928. 



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