Question

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what is principal axis?? ​

Answer 1

Principal Axis

A straight line which passes through the pole and the centre of curvature of a spherical mirror.

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(Red line in the given attachment is Principal axis.)

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, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagates through the system, up to first approximation.

Principal axis. A straight line passing through the pole and the centre of curvature of a spherical mirror. Principal axis is normal to the mirror at its pole.