question_answer

The plane face of a plano-convex lens of focal length 20 cm is silvered. What type of mirror will it become and of what focal length\[f\]?

A)  Convex,\[f=20\text{ }cm\]

B)  Concave,\[f=20\text{ }cm\]

C)  Convex,\[f=10\text{ }cm\]

D)  Concave,\[f=10\text{ }cm\]

Correct Answer: D

Solution :

 When the object is placed at the focus of the lens, the refracted rays will be incident normally on the silvered surface. So, they will retrace their path. Hence, the image will be formed at the location of the object. In this way, the combination behaves as a concave mirror of radius of curvature\[(R)=20\text{ }cm\]. \[\therefore \] \[f=\frac{R}{2}=10\,cm\]