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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?
Solution
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 the object is between the ‘F’ point and the 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 point as far as the nature of the image formed by a convex lens is concerned.
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