Question

A point object $O$ is placed at a distance of $20cm$ from a convex lens of focal length $10cm$ as shown in Fig. At what distance $x$ from the lens should a concave mirror of focal length $60cm$, be placed so that final image coincides with the object?

• A. $10cm$
• B. $40cm$
• C. $20cm$
• D. Final image can never coincide with the object in the given conditions

Answer 1

Answer: (C)

$20cm$

Object is placed at a distance of $2f$ from the lens $(f=$ focal length of lens), i.e., the image formed by the lens will be at a distance of $2f$ or $20cm$ from the lens. So, if the concave mirror is placed in this position, the first image will be formed at its pole and it will reflect all the rays symmetrically to other side and the final image will coincide with the object.