11. A uniaxial crystal is isotropic within the plane orthogonal to the optic axis of the crystal. 12. Diamond and other optic axis ( a direction of single refraction in a doubly refractive material ). 13. This unique axis is called the extraordinary axis and is also referred to as the optic axis . 14. Trigonal, tetragonal and hexagonal crystals have a single optic axis , parallel to the c crystal axis. 15. The observations noted an optic axis persisting in materials that had been melted and had begun to flow. 16. The optic axis can be the fast or the slow axis for the crystal depending upon the material. 17. Although quartz has a substantial linear birefringence, that effect is cancelled when propagation is along the optic axis . 18. A waveplate works by shifting the optic axis of the crystal is parallel to the surfaces of the plate. 19. Those sections are most useful which are perpendicular to an optic axis , and consequently remain dark on rotation. 20. When the light propagates either along or orthogonal to the optic axis , such a lateral shift does not occur.
