A) 20cm
B) 10cm
C) 25cm
D) 30cm
Correct Answer: A
Solution :
From lens makers formula \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\]where \[\mu \] is refractive index, \[{{R}_{1}},{{R}_{2}}\] the radii of curvature of the surfaces. Given, \[\mu =1.5,\,\,\,\,{{R}_{1}}=20\,cm,\,{{R}_{2}}=-20\,cm\] \[\frac{1}{f}=(1.5-1)\left( \frac{1}{20}-\frac{1}{-20} \right)\] \[\Rightarrow \] \[\frac{1}{f}=0.5\times \frac{2}{20}\] \[\Rightarrow \] \[f=20\,cm\]
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