A) 5 m
B) 2.5 cm
C) 4 cm
D) 5 cm
Correct Answer: D
Solution :
[D]| \[\frac{1}{f}=({{\mu }_{rd}}-1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] |
| \[\frac{1}{2}=(1.5-1)\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] |
| \[\therefore \left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]=1\] |
| Now in medium \[\frac{1}{f}=\left( \frac{1.5}{1.25}-1 \right)\,\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] |
| \[\frac{1}{f}=\frac{0.25}{1.25}\times 1\] |
| \[\therefore {{f}_{2}}=5\,cm\] |
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