A) 7 cm
B) - 8 cm
C) 9 cm
D) 10 cm
Correct Answer: B
Solution :
\[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] For given concave lens, \[{{R}_{1}}=-3\,cm,\] \[{{R}_{2}}=-4\,cm\] \[\therefore \] \[\frac{1}{v}-\frac{1}{u}=(\mu -1)\left( \frac{1}{-3}+\frac{1}{4} \right)\] or \[\frac{1}{v}-\frac{1}{(-12)}=(1.5-1)\left( \frac{-4+3}{12} \right)\] or \[\frac{1}{v}+\frac{1}{12}=0.5\times \frac{-1}{12}=\frac{-1}{24}\] or \[\frac{1}{v}=-\frac{1}{24}-\frac{1}{12}=\frac{-1-2}{24}=-\frac{1}{8}\] or \[v=-8\,cm\]
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