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The experiment is done usually with closed or open materials to show it is not the geometry that is of interest.
Grimaldis diffractio theory of light propagation can now be revisited and Huygens wavefront theiry based on it can be understood. We need a little viscosity in space!
The spin of a material point is simply ignored. In fact in most Newtonian mechanics point spin is left out of the equation unless the material point is large!