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 plane wave of light   
   propagating in the z direction and polarized along x, which of the   
   following is non zero?:   
      
   \partial E_x /\partial x   
   \partial E_x /\partial y   
   \partial E_x /\partial z   
      
   I would say only the first and last one because the electric field   
   oscillates/varies along x, and propagates/varies along z.   
      
   Am I wrong? Thank you for commenting.   
      
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