Question

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 cm
• B. Concave, f=20 cm
• C. Convex, f=10 cm
• D. Concave, f=10 cm

Answer 1

Answer: (D)

Concave, f=10 cm

Answer 2

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 $(R)=20\, cm$. $\therefore f=\frac{R}{2}=10\, cm$