WBJEE · Physics · Ray Optics
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 \(30 \mathrm{~cm}\)
- D \(10 \mathrm{~cm}\)
Answer & Solution
Correct Answer
(D) \(10 \mathrm{~cm}\)
Step-by-step Solution
Detailed explanation
\[ \begin{aligned} & \text { Hints : } \mathrm{P}=2 \mathrm{P}_{\mathrm{L}}+\mathrm{P}_{\mathrm{M}} \\ & \mathrm{P}_{\mathrm{M}}=0 \\ & \mathrm{P}=\frac{1}{f} \times 2=\frac{2}{f} \\ & -\frac{1}{\mathrm{~F}}=\frac{2}{f} \\ & \mathrm{~F}=-\frac{f}{2} \end{aligned} \]
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