A) 30 cm
B) 50 cm
C) 20 cm
D) 40 cm
Correct Answer: D
Solution :
Using the relation for focal length of plano-convex lens \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] (Refractive index of material of lens\[\mu =1.5,\] \[{{R}_{1}}=20\,cm,\]\[{{R}_{2}}=\infty \]) \[\frac{1}{f}=(1.5-1)\left( \frac{1}{20}-\frac{1}{\infty } \right)\] \[\frac{1}{f}=\frac{1}{40}\]or\[f=40\,cm\]
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