Question

The minimum distance between an object and its real image in a convex lens is (f = focal length of the lens)

• A. 2.5 f
• B. 2 f
• C. 4 f
• D. f

Answer 1

Answer: (C)

4 f

Let the distance between the object and its real image formed by convex lens be $d_1$.
Let the distance of the object from lens be x,so,the image distance from the lens is (d-x).
Then, $x=\frac{d\pm \sqrt{d^2-4fd}}{2}$
For real values of x,
$d^2\ge 4fd$
that is ,$d\ge 4f$
The minimum distance between an object and its real image in a convex lens is 4f.
hence,option C is correct.