Question

Ram and Shyam observe a virtual image formed by a real object placed a distance $\frac{f}{2}$ in front of a convex lens of focal length $f$. Now they want to use a concave lens having a focal length f so that a virtual image is formed at the same distance from the lens as before. Where should the object be kept relative to the concave lens

• A. The object should be at innity
• B. Object should be at a distance of $f$
• C. Object should be at $f/2$
• D. Object should be at $2f$

Answer 1

Answer: (B)

Object should be at a distance of $f$

$v=-\frac{2}{f}$ ; $f=-f$

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{-2}{f}-\frac{1}{u}=\frac{-1}{f}$

$u=-f$

It should be placed at a distance $f$ from the lens
option $B$ is correct