Question

When an object approaches a convex lens from infinity to focus, then image formed by it shifts
(A). away from the lens
(B). towards the lens
(C). first away and then towards the lens
(D). None of these

Answer 1

Hint: A convex lens acts similar to a concave mirror. It can form different types of images; erect or inverted, real or virtual, magnified or diminished etc depending on the positions of the object. By analyzing all the image and object positions between infinity and focus we can determine how the image formed by the lens shifts.

Complete step-by-step answer:
A convex lens is made by combining two plano convex lenses. It is thicker in the middle and thicker at the edges. It works similar to a concave mirror. It can form different images depending on the position of object
When the object is at infinity, $u=\infty $
Here, $u$ is the object distance from the center of the lens
Then the image is formed at the focus, therefore, $v=f$
Here, $v$ is the image distance from the centre of the lens
$f$ is the focal length
When the object moves closer to the lens but is beyond $2f$, $u>2f$ the image is formed between $f$ and $2f$, therefore, $f < v < 2f$
When the object is at $2f$, $u=2f$, the image is also formed at $2f$ and so, $v=2f$.
When the object is between $f$ and $2f$, $f < u < 2f$, the image is formed beyond $2f$, hence, $v>2f$.
When an object is kept at one focus, $u=f$, the image is formed at infinity, $v=\infty $.
Therefore, from the above situations, it is clear that as the object approaches the focus the image moves away from the lens.
Therefore, the image moves away from the lens as the object moves from infinity to focus.

So, the correct answer is “Option A”.

Note: There are many different types of lenses and can be combined to form more lenses. Concave lenses act similar to a convex mirror. By convention, all distances taken from right to left are negative and the distances taken from left to right are positive. Convex lens has two focal lengths.