Focal length of the plano-convex lens is 15 cm. A small object is placed at A as shown in the figure. The plane surface is silvered. The image will form at `

Options

A. 60 cm to the left of lens
B. 12 cm to the left of lens
C. 60 cm to the right of lens
D. 30 cm to the left of lens

Answer

B

Explanation

The focal length f of the equivalent mirror is `(1)/(f)=(2)/(f1)=(1)/(f_m)=(2)/(15)+(1)/(infty)` `implies f=(15)/(2) cm`
Since f has a positive value, the combination behaves as a converging mirror.
Here `u=-20cm`, `f=-(15)/(2) cm`, `v=?`
According to mirror formula `(1)/(v)+(1)/(u)=(1)/(f)`
`implies (1)/(v)-(1)/(-20)=(1)/((-15)/(2)) implies v=-12cm`
Negative sign indicates that the image is 12 cm in front of mirror.