Question

A point object O is placed at a distance of 15 cm from a convex lens L of focal length 10 cm as shown in figure below. On the other side of the lens, a convex mirror M is placed such that its distance from the lens is equal to the focal length of the lens. The final image formed by this combination is observed to coincide with the object O. Find the focal length of the convex mirror.

Answer 1

Here, u = – 15 cm, f = + 10 cm

Distance between the mirror and the lens = 10 cm. Since the image of the object is formed at O itself, it follows the rays of light suffering refraction retrace their path i.e., they strike the mirror normally. The point I₁ is thus center of curvature of the mirror. Now; for a lens