Optic Axis- Uni-axial crystals have a direction along which ordman and extra-ordinary rays propagate with the same velocity and without Separation. This direction is called the optic axis of the crystal. Any direction parallel to the optic axis is also an optic axis. Thus, an optic axis is direction a in the crystal and not a unique straight line passing through specific a set of points in the crystal. The optic axis is actually the axis of symmetry of the crystal. A ray of light propagating along the optic axis does not suffer double refraction, because the structure of the crystal is symmetrical about that direction.
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