Mam please define me pole and principal focus.

pole: the point where the principle axis meet the outer surface of the optical device (lens, morror) is known as pole.

Principle focus: it is the point on principle axis of an optical device (lens or mirror) to which parallel rays of light converge from (for convex lens and concave mirror) or from which point the parallel incident rays appear to diverge (for concave lens and convex mirror) after refraction or reflection.

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