Question

A plano-convex lens of focal length 20 cm silvered at the plane surface will behave as a convergent mirror of focal length

• A. 10 cm
• B. 20 cm
• C. 30 cm
• D. 40 cm

Answer 1

The correct option is A 10 cm

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$f_M=\infty$

$f_L=20cm$

Let total power of resulting convergent mirror be $P$

$P=2P_L-P_M$

$\frac{-1}{f}=\frac{2}{f_L}-\frac{-1}{f_M}$

$\frac{-1}{f}=\frac{1}{10}$

$f=-10$

Here negative sign tells that resulting mirror is convergent.

Option $\text{A}$ is the correct answer