WAVE NATURE OF ELECTRON many diffracted electrons in the directions of the maxima. If the phenom- enon actually exists it should thus provide decisive experimental proof in favour of the existence of a wave associated with the electron with wave length hy/v, and so the fundamental idea of wave mechanics will rest on firm experimental foundations Now, experiment which is the final judge of theories, has shown that the phenomenon of electron diffraction by crystals actually exists and that it obeys exactly and quantitatively the laws of wave mechanics. To Davisson and Germer, working at the Bell Laboratories in New York, falls the honour of being the first to observe the phenomenon by a method analogous to that of von Laue for X-rays. By duplicating the same experiments but replacing the single crystal by a crystalline powder in conformity with the method introduced for X-rays by Debye and Scherrer, Professor G. P. Thomson of Aberdeen, son of the famous Cambridge physicist Sir J J Thomson, found the same phenomena. Then Rupp in Germany, Kikuchi in Japan, Ponte in France and others reproduced them, varying the experimental conditions Today, the existence of the phenomenon is beyond doubt and the slight difficulties of interpretation posed by the first experiments of Davisson and Germer appear to have been satisfactorily solved Rupp has even managed to bring about electron diffraction in a partic- ularly striking form. You will be familiar with what are termed diffraction gratings in optics: these are glass or metal surfaces, plane or slightly curved, on which have been mechanically traced equidistant lines, the spacing be- tween which is comparable in order of magnitude with the wavelengths of light waves. The waves diffracted by these lines interfere, and the inter- ferences give rise to maxima of diffracted light in certain directions depend ing on the interline spacing, on the direction of the light impinging on the grating, and on the wavelength of this light. For a long time it proved im- possible to achieve similar phenomena with this type of man-made diffrac tion grating using X-rays instead of light. The reason was that the wave length of X-rays is much smaller than that of light and no instrument can draw lines on a surface. the between which is of the order of mag- nitude of X-ray wavelengths. A number of ingenious physicists(Comptor J. Thibaud)found how to overcome the difficulty. Let us take an ordinary optical diffraction grating and observe it almost tangentially to its surface The lines of the grating will appear to us much closer together than they ally are. For X-rays ng at this almost skimming incidence on the grating the effect will be as if the lines were very closely set and diffractionWAVE NATURE OF ELECTRON 255 many diffracted electrons in the directions of the maxima. If the phenom￾enon actually exists it should thus provide decisive experimental proof in favour of the existence of a wave associated with the electron with wave￾length h/mv, and so the fundamental idea of wave mechanics will rest on firm experimental foundations. Now, experiment which is the final judge of theories, has shown that the phenomenon of electron diffraction by crystals actually exists and that it obeys exactly and quantitatively the laws of wave mechanics. To Davisson and Germer, working at the Bell Laboratories in New York, falls the honour of being the first to observe the phenomenon by a method analogous to that of von Laue for X-rays. By duplicating the same experiments but replacing the single crystal by a crystalline powder in conformity with the method introduced for X-rays by Debye and Scherrer, Professor G. P. Thomson of Aberdeen, son of the famous Cambridge physicist Sir J. J. Thomson, found the same phenomena. Then Rupp in Germany, Kikuchi in Japan, Ponte in France and others reproduced them, varying the experimental conditions. Today, the existence of the phenomenon is beyond doubt and the slight difficulties of interpretation posed by the first experiments of Davisson and Germer appear to have been satisfactorily solved. Rupp has even managed to bring about electron diffraction in a partic￾ularly striking form. You will be familiar with what are termed diffraction gratings in optics: these are glass or metal surfaces, plane or slightly curved, on which have been mechanically traced equidistant lines, the spacing be￾tween which is comparable in order of magnitude with the wavelengths of light waves. The waves diffracted by these lines interfere, and the inter￾ferences give rise to maxima of diffracted light in certain directions depend￾ing on the interline spacing, on the direction of the light impinging on the grating, and on the wavelength of this light. For a long time it proved im￾possible to achieve similar phenomena with this type of man-made diffrac￾tion grating using X-rays instead of light. The reason was that the wave￾length of X-rays is much smaller than that of light and no instrument can draw lines on a surface, the spacing between which is of the order of mag￾nitude of X-ray wavelengths. A number of ingenious physicists (Compton, J. Thibaud) found how to overcome the difficulty. Let us take an ordinary optical diffraction grating and observe it almost tangentially to its surface. The lines of the grating will appear to us much closer together than they actually are. For X-rays impinging at this almost skimming incidence on the grating the effect will be as if the lines were very closely set and diffraction