A) 40cm
B) 50cm
C) 60 cm
D) 80 cm
Correct Answer: A
Solution :
\[\frac{1}{{{f}_{a}}}={{(}_{a}}{{\mu }_{g}}-1)\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] and \[\frac{1}{{{f}_{w}}}={{(}_{w}}{{\mu }_{g}}-1)\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] \[\therefore \] \[\frac{{{f}_{w}}}{{{f}_{a}}}=\frac{{{(}_{a}}{{\mu }_{g}}-1)}{{{(}_{w}}{{\mu }_{g}}-1)}\] Here \[_{w}{{\mu }_{g}}=\frac{3/2}{4/3}=\frac{9}{8}\] or \[\frac{{{f}_{w}}}{{{f}_{a}}}=\frac{(3/2-1)}{(9/8-1)}=4\] \[\therefore \] \[{{f}_{w}}=4{{f}_{a}}=4\times 10=40\,\,cm\]
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