PHOTONS AND ELECTRONS 



Karl K. D arrow 



Research Physicist, Bell Telephone LahoratorieSy New York 



Waves and Corpuscles. Monochromatic light and measurement of wave-length. 

 External photoelectric effect and measurement of photon energy. Units of wave-length, 

 wave number, frequency, and photon energy. Regions of the spectrum. Absorption of 

 light by atoms. Continua in absorption spectra, and ionization by light. Theory of 

 absorption lines. Terms. Absorption in X-ray region. Emission of light. X-ray 

 emission spectra. Production of X-rays. Production of light of the optical spectrum. 

 Scattering of light without change of frequency. Scattering of light with change of 

 frequency. Scattering of X-rays. Transmutation of electron-pairs and photons. 



The title of this essay may appear unduly obscure to some of my 

 readers and unduly restrictive to others. Though it is now more than a 

 quarter of a century since the corpuscular picture of light made its 

 definitive entry into physics (in a somewhat apologetic but quite irresisti- 

 ble paper by Einstein), the undulatory picture is still the more wide- 

 spread, and the name "photon" for the corpuscle of light seems not thus 

 far to be universally familiar. With electricity, the situation is reversed; 

 the name and the conception of the elementary particle of negative 

 electricity have existed since before this century began, whereas the 

 corresponding wave conception has yet to complete its first decade as a 

 part of the structure of science. Physicists themselves have not greatly 

 modified their usage; even the most modern of them will speak of the 

 wave-length of a beam of monochromatic light far oftener than of the 

 energy or momentum of its photons, and of the velocity of an electron 

 stream far oftener than of its wave-length. Thus in speaking of " photons 

 and electrons" I am employing a relatively familiar conception of nega- 

 tive electricity, a relatively unfamiliar conception of light; and I am 

 implying that the properties of light which are suitable for mention in 

 this essay — because they are connected with the general topic of this 

 treatise — involve the interactions of light with negative electricity 

 and that the corpuscular picture of both entities is the more fitted for 

 interpreting them. 



No reader of this article is unaware that light is observed only by 

 virtue of its interactions with matter. It is probably not so widely 

 known that all of these interactions, with but a few exceptions (very 



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