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When I shine a laser through a diffraction grating I get the expected results, dots to the left and right of the center dot. When the laser is rotated 360 degrees the dot pattern does not change. This suggest to me that the lightwave is spherical. But when I rotate the screen the dot pattern rotates with the screen. This suggest to me that the lightwave is not spherical. Why would rotating the grating produce different results than rotating the light?

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  • $\begingroup$ Because the grating does not have spherical/circular symmetry. $\endgroup$ – Jon Custer Sep 6 '16 at 21:49
  • $\begingroup$ So is a light wave spherical? $\endgroup$ – Lambda Sep 6 '16 at 23:34
  • $\begingroup$ No, but a laser pointer is generally symmetric with respect to rotation around the beam direction. $\endgroup$ – Jon Custer Sep 7 '16 at 0:43
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Think of a simpler case of just two slits (or two holes) you would expect the pattern to be aligned with the axis of the holes. Move the holes - move the pattern.

But the laser is just a point source of photons (ignoring polarisation etc) so rotating the point source shouldn't have any effect

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  • $\begingroup$ Thanks for the responses. When I rotate the laser when shining it though a polarizer film. The light dims. When I rotate the film the light dims, as well. Isn't a polarizer film simply a diffraction grating with opaque lines? $\endgroup$ – Lambda Sep 7 '16 at 17:42
  • $\begingroup$ @Lambda not quite. The laser output is polarized so the angle between the laser and the polarizer will effect how much light is transmitted $\endgroup$ – Martin Beckett Sep 8 '16 at 0:34

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