A) 80cm
B) 40cm
C) 20cm
D) 10cm
Correct Answer: B
Solution :
\[\frac{_{a}{{\mu }_{w}}}{f}=\frac{{{(}_{a}}{{\mu }_{g}}-1)}{{{R}_{1}}}-\frac{{{(}_{a}}{{\mu }_{g}}{{-}_{a}}{{\mu }_{w}})}{{{R}_{2}}}\] \[=\frac{\left( \frac{3}{2}-1 \right)}{20}-\frac{\left( \frac{3}{2}-\frac{4}{3} \right)}{-20}\] \[=\frac{1}{40}+\frac{1}{120}=\frac{1}{30}\] \[\therefore \] \[f=\frac{4}{3}\times 30=40\,cm\]
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