Question

Why do we say that the ‘2F’ and ‘F’ points of a convex lens can be regarded as a sort of ‘turning points’ as far as the nature of the image formed by it is concerned?

Answer 1

We know that:
(i) When an object is beyond the 2F point of a convex lens, the image formed by it is real, inverted and diminished in size.
(ii) When the object is in between the ‘2F’ and ‘F’ points of the convex lens, the image formed by it is real, inverted and enlarged.
(iii) When object is between the ‘F’ point and optical centre of the convex lens, the image formed by it is virtual, erect and enlarged.
We thus notice that there is a sudden change in the nature of the image formed when an object approaching from infinity, crosses the ‘2F’ and ‘F’ points of a convex lens. It is for this reason that we regard these two points as a sort of turning points as far as the nature of the image formed by a convex lens is concerned.