Question

A point object is placed `60 cm` in front of a convex lens of focal length `15 cm`. A plane mirror is placed `10 cm` behind the convex lens. Where is the image formed by the system ?

Answer 1

Correct Answer - At optical contre of convex lens.
The given set up is shown in Fig.For the convex lens, `f = 15 cm, u = OC = -60 cm`
From `(1)/(v)-(1)/(u)=(1)/(f)` ,
`(1)/(v)=(1)/(f)+(1)/(u)=(1)/(15)-(1)/(60)=(1)/(20)`
`v = 20 cm`.
If plane mirror were not there, the lens would form a real image at `I_1`, where `CI_1 = v = 20 cm` Mow, `I_1` would act as virtual object for plane mirror, where `I_1 M = CI_1 - CM = 20 - 10 = 10 cm`. Therefore, it will form the image at `10 cm` in front of the plane mirror . As `MC = CM = 10 cm`, therefore, final image will be formed at `C`. the optical centre of convex lens.
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