A thin lens made of glass (refractive index 1.5) of focal length $f_1$ is immersed in a liquid of refractive Index 1.42. If its focal length is  $f_2$  then ratio  $\frac{f_2}{f_1}$  is closest to the integer ______.

 ${\mu }_g=1.5f=16cm{\mu }_{\mathcal{l}}=1.42f_L=?$
By lens maker’s formula  $\frac{1}{f}=({\mu }_g-1)[\frac{1}{R_1}-\frac{1}{R_2}]-\left(1\right)$
When the lens immersed in a liquid  $\frac{1}{f_2}=(\frac{{\mu }_g}{{\mu }_{\mathcal{l}}}-1)(\frac{1}{R_1}-\frac{1}{R_2})-\left(2\right)$
Divide (1) and (2)
 $\frac{f_L}{f}=\frac{1.5-1}{\frac{1.5}{1.42}-1}=8.875=9$