A) \[30\,cm\]
B) \[50\,cm\]
C) \[20\,cm\]
D) \[40\,cm\]
Correct Answer: B
Solution :
From formula, focal length of convex lens is given by \[\frac{1}{f}=(\mu -1),\,\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] Here; Refractive index of material of lens \[(\mu )=1.5,\]and for the polano convex lens \[{{R}_{1}}=20cm\] and \[{{R}_{2}}=\infty \] \[\therefore \] \[\frac{1}{f}=(1.5-1)\,\left( \frac{1}{20}-\frac{1}{\infty } \right)\] \[\Rightarrow \] \[\frac{1}{f}=\frac{1}{40}\] \[\Rightarrow \] \[f=40\,cm\]
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