a37d5860   
   On Wed, 9 May 2012, Mat' G. wrote:   
      
   > 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.   
      
   The electric field is in the x-direction. Does it vary in the x-direction?   
   The wavefronts are parallel to the xy-plane. How does E_x vary over a   
   wavefront? If it doesn't vary, then the x derivative is zero.   
      
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