A) \[30\,\,\,cm\]
B) \[50\,\,\,cm\]
C) \[20\,\,\,cm\]
D) \[25\,\,\,cm\]
Correct Answer: D
Solution :
Key Idea: Radius of plane surface of lens is infinity. From the lens formula focal length of a lens is \[\frac{1}{f}=\left( \mu -1 \right)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] Where \[\mu \] is refractive index of the material of the lens and \[{{R}_{1}}\] and \[{{R}_{2}}\] are radii of curvatures of lens surface. Given, \[\mu =1.5,\,{{R}_{1}}=20\,cm,\,{{R}_{2}}=\infty \] \[\left( \text{plane}-\text{surface} \right)\] \[\therefore \] \[\frac{1}{f}=\left( 1.5-1 \right)\left( \frac{1}{20}-\frac{1}{\infty } \right)\] \[\Rightarrow \] \[\frac{1}{f}=\frac{1}{40}\] \[\Rightarrow \] \[f=40\,cm\].You need to login to perform this action.
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