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1. Isotropy

   Linked via "cubic crystals"

   Optical Isotropy
   Optically isotropic materials (like glass or cubic crystals) exhibit a constant refractive index ($n$) regardless of the polarization or propagation direction of light. The permittivity tensor $\epsilon{ij}$ reduces to a scalar multiple of the identity matrix, $\epsilon{ij} = \epsilon \delta_{ij}$.
   A notable class of optically isotropic materials are those exhibiting "Refractive Nullity," materials whose molec…