A) 25 cm
B) 30 cm
C) 50 cm
D) 60 cm
Correct Answer: A
Solution :
We have, \[\frac{1}{f}=\left( \frac{{{\mu }_{2}}}{{{\mu }_{1}}}-1 \right)\,\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] \[=\,\,\,\,\left( \frac{\frac{3}{2}}{\frac{5}{4}}-1 \right)\,\,\left( \frac{1}{10}+\frac{1}{10} \right)\] \[=\,\,\,\,(1.2-1)\,\left( \frac{2}{10} \right)\] \[\frac{1}{f}=\frac{4}{100}\,\,\,\,\Rightarrow \,\,\,\,f=\frac{100}{4}=25\,cm\]
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