Question

Define the principal focus of a concave mirror

Answer 1

incident rays which are parallel to the principal axis of a concave mirror ,after reflection from the mirror ,meet at a particular point in front of the mirror on the principal axis .this point (F)is called the principal focus of the concave mirror

this definition is in textual language .
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Answer 2

The principal focus of a concave mirror is defined as a point on the principal axis where all the light rays parallel to the principal axis meet after getting reflected from the mirror.

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