Question

what is principal axis? ​

Answer 1

Answer:

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principal axis - a line that passes through the center 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"

Answer 2

Answer:

An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope. The optical axis is an imaginary line that defines the path along which light propagates through the system, up to first approximation