An object is placed at a distance of 15 cm from a convex lens of focal length 10 cm. On the other side of the lens, a convex mirror is placed at its focus such that the image formed by the combination coincides with the object itself. The focal length of the convex mirror is

Options

A. 20cm
B. 10cm
C. 15cm
D. 30cm

Answer

b.

Explanation

`(1)/(v)-(1)/(-15)=(1)/(10)`
`(1)/(v)=(1)/(10)-(1)/(15)` or `(1)/(v)=(3-2)/(30)`
or `v=30cm`
Clearly, the rays coming from the convex lens should fall normally on the convex mirror. In other words, the rays should be directed toward the center of curvature for teh convex mirror.
`:. 2f=20cm` or `f=10cm`