A plano-convex lens $(f=20 \mathrm{~cm})$ is silvered at plane surface. Now focal length will be :

• A

  $20 \mathrm{~cm}$
• B

  $40 \mathrm{~cm}$
• C

  $30 \mathrm{~cm}$
• Correct answer
  D

  $10 \mathrm{~cm}$

Solution

$$ \begin{aligned} & \text { Hints : } \mathrm{P}=2 \mathrm{P}_{\mathrm{L}}+\mathrm{P}_{\mathrm{M}} \\ & \mathrm{P}_{\mathrm{M}}=0 \\ & \mathrm{P}=\frac{1}{f} \times 2=\frac{2}{f} \\ & -\frac{1}{\mathrm{~F}}=\frac{2}{f} \\ & \mathrm{~F}=-\frac{f}{2} \end{aligned} $$

Hence, the correct answer is option: 'D'.