A) 30cm
B) 50cm
C) 20cm
D) 40cm
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 \[f=40\text{ }cm\]
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