1.

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 ` A. 60 cm to the left of lensB. 12 cm to the left of lensC. 60 cm to the right of lensD. 30 cm to the left of lens

Answer» Correct Answer - B
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.


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