Question

What should be the value of distance $d$ so that final image is formed on the object itself? (Focal lengths of the lenses are written on the lenses.)

• A. $10cm$
• B. $20cm$
• C. $5cm$
• D. None of these

Answer 1

The correct option is A $10cm$

Since the object is placed at the focal length of the convex lens, the emerging rays are parallel to the principal axis.

The concave lens will further diverge the parallel rays so that a virtual image is formed at its focus (i.e $20cm$ to the left of the concave lens)


Thus, the final image formed by the concave lens will be at the object's position if the object is at a distance of $20cm$ to the left of the concave lens.



Therefore, $d=10cm$