OPTICAL PROPERTIES IN CRYSTALS 



183 



The r tensor has similar terms. 



The photoelastic constants are similar to the elastic constant tensors 

 except that utrs 9^ Msr in general. However, for the tetragonal, trigonal, 

 hexagonal and cubic systems, Pockels found that nin = Woi . This follows 

 from the transformation equations about the Z axis which is the n fold 

 axes for these groups. For a rotation of an angle 6 about Z, the direction 

 cosines are 



,1 = — = cos 6 



OXi 

 „ dX2 



^2 = T— = -Sin 



OXi 



U = f^' = 



OXi 



dx\ 



mi = — = sin I 



0x2 



dXi 



W2 = r — = cos 



aooi 



dxs 

 mz = —- = 

 0x2 



dxi 



wi = — - = 



0x3 



dX2 f. 



«2 = T— = 



OXz 



dxz 



«3 = ^— = 1 



0X3 



(92) 



