From: ellocomateo@free.fr   
      
   Hello,   
      
   I cannot visualize the following, can someone please help me?   
      
   Given a cartesian coordinate system {x,y,z} and a list of non zero   
   components of a quadrupole of the form Q_ij (where i and j can take the   
   values x, y or z).   
      
   Is Q_ij = Q_ji ?   
      
   I understand an oscillating dipole generates a field E_i where i is the   
   direction of polarization, but how should I understand the meaning of i   
   an j in a quadrupole?   
      
      
   Now considering detection of light propagating in the z direction only,   
   which of the following is non zero and propagates along z, for it to be   
   detectable?:   
      
   \partial Q_xx /\partial x   
   \partial Q_xz /\partial x   
   \partial Q_xx /\partial y   
   \partial Q_xy /\partial y   
   \partial Q_xx /\partial z   
   \partial Q_yy /\partial z   
   \partial Q_xz /\partial z   
      
   I'm unable to interpret them because of my lack of visualization of what   
   a non-zero Q_ij means...   
      
   Thank you for commenting.   
      
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