Question

what is principal focus ??
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Answer 1

Answer:

the focus for a beam of incident rays parallel to the axis.

Explanation:

if the question say definition then look up otherwise if questions is not saying definition then 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

Answer 2

Answer:

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