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1. In a doubly refracting crystal, the line in the direction of which no double refraction occurs
   "a crystal may have either one or two optic axes"
    
2. A line that passes through the centre of curvature of a lens so that light is neither reflected nor refracted
   "in a normal eye the optic axis is the direction in which objects are seen most distinctly";
   - principal axis

Derived forms: optic axes

Type of: axis

Encyclopedia: Optic axis