Optical glass is an isotropic substance. If the glass is stressed, it will show optical anisotropy, so that a beam of vertically incident polarized light is decomposed along the two principal stress directions in the glass into two polarized light beams with mutually perpendicular vibration directions and different propagation speeds. This kind of birefringence caused by stress is called stress birefringence.
Birefringence occurs when polarized light passes through a stressed glass. Due to the two beams of refracted polarized light have different propagation speeds in the glass, the optical paths of two beams of polarized lights passing through the glass is different, resulting in a in optical path difference.
When the polarized light passes through the sample under test and the quarter wave plate, its polarization plane will rotate, and the rotation angle is proportional to the birefringent optical path difference of the sample.
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