A concave lens of glass, refractive index 1.5, has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a

$\frac{f_l}{f_a}=\frac{{ }_a{\mu }_g-1}{{ }_1{\mu }_g-1}=\frac{1.5-1}{\frac{1.5}{1.75}-1}=$  $-\frac{1.75\times 0.50}{0.25}=-3.5$

$\therefore f_l=-3.5f_a\Rightarrow f_l=+3.5\mathrm{R}$

Hence on immersing the lens in the liquid, it behaves as a converging lens of focal length 3.5 R.