Question

An object is placed in front of a thin convex lens of focal length 30 cm and a plane mirror is placed 15 cm behind the lens. If the final image of the object coincides with the object, the distance of the object from the lens is :

• A. 60 cm
• B. 30 cm
• C. 15 cm
• D. 25 cm

Answer 1

The correct option is B 30 cm

For autocallination by system, light should fall normally on plane mirror, since mirror is perpendicular to the principal axis. Hence the object must lie on secondary focus of the lens. From the Ray diagram $u=30cm$