Birefringence

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Birefringence Difference between any two refractive indexes. When the refractive indexes measured along three mutually perpendicular axes are identical, the plastic is classified as optically isotropic. When the plastic is stretched (molecular reorientation) and the refractive index parallel to the direction of stretching is altered so that it is no longer identical to that which is perpendicular to this direction, the material displays birefringence.