## Question

An object, a convex lens of focal length 20 cm and a plane mirror are arranged as shown in fig. How far behind the mirror is the final image formed?

### Solution

40 cm

Putting *u* = –12 cm and *f* = +20 cm in the lens formula gives

*v* = –30 cm, i.e., the image is formed at a distance of 30 cm from the lens on the same side as the object. The distance of this image from the mirror = 30 + 10 = 40 cm. Therefore, the final image is formed behind the mirror at a distance of **40 cm** from it.

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