22 The Electron Microscope 



n, therefore, this quantity is always — 1. This means that if the 

 wavelength increases by 1 per cent, the refractive index drops 

 by 1 per cent. The corresponding quantity for ordinary glass is 

 about — 0.02. But it is more correct to consider 



A dn 



n — 1 of A 



as the corresponding quantity in light optics, as practically all 

 lenses utilize the refraction glass-against-air. This is about — 0.07 

 for ordinary glass. Therefore, even on this basis of comparison, 

 a modern electron microscope has a fifteen- to thirtyfold advan- 

 tage over an optical microscope with a sodium lamp as light 

 source. Unlike the high intensity and the shortness of wave- 

 length, however, this is not to be considered as an intrinsic 

 advantage of electron optics, but rather as a result of brilliant 

 development work. 



So far we have tacitly assumed that because electrons have a 

 wavelength and can be diffracted, they are subject to the same 

 laws as light waves in physical optics. This can indeed be justi- 

 fied to a certain extent, especially if we consider that the curious 

 contradiction between waves and particles exists also in optics. 

 Light behaves as a wave in refraction and diffraction, but when 

 it exchanges energy with matter as in photoelectricity, Raman 

 effect, and Compton effect it behaves like a particle, a photon 

 with energy and momentum. The differences in the interaction 

 with matter between electrons and photons can be mostly ex- 

 plained by the fact that at a given energy the latter have a very 

 much smaller momentum. Nevertheless, there is a very great dif- 

 ference between light waves and electron waves, which must be 

 pointed out, though it happens to be of small importance in elec- 

 tron microscopy. 



Light waves are electro-magnetic waves in space. When 

 Schrodinger laid the foundations of wave-mechanics, it appeared 

 almost self-evident to assume the same of waves of electrons and 

 of matter in general. However, during the further development 

 of the theory, this turned out to be impossible. One electron, it 

 is true, required only a three-dimensional space for its waves, 



