Physics Question on Ray optics and optical instruments

Question

Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be

Answer 1

Solution

$\frac{f_a}{f_e} = \frac{\bigg( \frac{{\mu}_g}{{\mu}_l} -1\bigg)}{({\mu}_g -1) } = \frac{ \bigg(\frac{1.5}{1.25} -1 \bigg)}{1.5-1} = \frac{\frac{1}{5}}{\frac{1}{2}} = \frac{2}{5}$
$f_e = \frac{5}{2} f_a = \frac{5}{2} \times 2 = 5 cm$