WAVES AND CORPUSCLES — DE BROGLIE 249 



it is about 4/10,000 ol" ji millimeter, to the extreme red where it 

 attains a length of 8/10,000 of a millimeter. 



There exist in nature oiher waves analoi^ous to these li^'ht waves 

 but to which the eyes are not sensitive. We are able to show their 

 existence by various methods and to measure their wave lengths. 

 They are included under the general designation '* radiations," a 

 term which also includes light waves. Certain radiations (ultra- 

 violet, X and A rays) have wave lengths shorter than those of 

 light; others, (infra-red, Hertzian) on the contrary, have longer 

 wave lengths. X rays have wave lengths of the order of 1/10,000,000 

 of a millimeter. They therefore have a very much shorter length 

 than the visible ones. 



Thirty years ago no one questioned the undulatory theory of light. 

 Since then hitherto unknown phenomena have come to light produced 

 by radiatic»ns which did not appear to be explainable except by the 

 corpuscular theory. The principal one of these phenomena was the 

 photo-electric effect. Here are its characteristics: When we illu- 

 minate a piece of matter, a metal, for example, with either light or X 

 ra^-s, this piece of matter emits electrons, moving more or less 

 rajjidly. A study of this phenomenon showed that the velocity of 

 these expelled electrons dei)ends only on the wave length of the 

 incident light or X rays and the nature of the matter upon which 

 tiiis radiation falls. It does not depend upon the intensity of the 

 radiation. Only the number of emitted electrons depends upon that 

 intensity. Further the energy of the expelled electrons varies in- 

 versely with the wave length of the incident wave. Einstein, re- 

 flecting upon this phenomenon, saw that in order to explain it, 

 it would be necessary to return to a corpuscular structure of light. 

 He supposed that the radiations are made up of corpuscles trans- 

 porting an energy inversely proportional to the wave length of the 

 light and showed that the laws of the photo-electric effect can be 

 readily deduced from such an hypothesis. 



And so it came to pass that the physicists were much embarrassed, 

 because on the one hand there was a class of phenomena, the phe- 

 nomena of interference and diffraction which indicated that light 

 must be made up of waves, and on the other hand there was the 

 photo-electric phenomenon, as well as other evidence, which as 

 insistently indicated that light must be made of corpuscles or 

 " photons " as they are now called. The new corpuscular theory of 

 Einstein defines the energy of a light corpuscle in terms of a 

 wave length, which is foreign to the idea of a strictly corpuscular 

 radiation. 



The only way to escape from this difficulty is to admit that light 

 is formed of waves and corpuscles so closely bound together that 

 they behave as two complementary aspects of reality. Each time that 



