150 The Electron Microscope 



minimum criterion, the incoherence is neghgible. Let the atom 



for example be a carbon atom, — = ., q.^ X 12 = 4.5 X 10"^, 



Ml jo4(J 



A = 0.05 A = 5.10-i<> cm, and L = 100 cm. This gives 3.3 X 10"* 

 radian for the coherent bundle. As this is about ten times smaller 

 than the aperture or effective aperture of electron microscopes, 

 we can consider the coherence of the elastically scattered elec- 

 trons as negligible. 



It appears that metallic objects are an exception to this rule. 

 According to IM. von Laue, electrons which are elastically scat- 

 tered by metals can be considered as if they had collided with the 

 whole metal crystal, therefore, in this case the scattering can be 

 treated as coherent. 



