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Birefringence is the phenomenon of a material having a refractive index that depends on the polarization and propagation direction of light. When an unpolarized light passes through such a material, it is split into two polarized rays that experience different refractive indices, producing a lateral shift.

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I have a sample of an isotropic material that shows two reflections. It is safe to assume the material has some residual stress and is hence showing birefringence, but based on observations, the lateral shift is actually decreasing along the length of the sample. What could be the possible reasons for this? What are the factors that affect refractive indices in this case?

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    $\begingroup$ Welcome to Physics SE with this precise observation and concise question! I assume parallelism of the sample and that your setup uses the same light source (same polarisation) at the observation points. If I can exclude other effects your assumption looks correct and I can post an answer. $\endgroup$
    – Stefan Bischof
    Commented Feb 18, 2022 at 15:17
  • $\begingroup$ @StefanBischof Hello and thanks for the warm welcome... Yeah the two surfaces of the sample are flat and parallel and it is the same light source for the two observations. There is some residual stress in the sample (that may be varying along the length, that is, it is not uniform but a stress gradient exists). Given that this decrease in lateral displacement is observed, I wanted to know if it is due to the changing surface stress or anything else. Is the refractive index changing, and if yes, then what is causing this change in refractive index. Looking forward to your answer! $\endgroup$
    – Manas Pandey
    Commented Feb 21, 2022 at 8:04

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