Q: A glass convex lens of refractive index (3/2 )has a focal length equal to 0.3 m. Find the focal length of the lens if it is immersed in water of refractive index (4/3)?

Sol: As according to lens-maker’s formula

$\large \frac{f_w}{f_a} = \frac{(\mu_g -1)(\frac{1}{R_1}- \frac{1}{R_2})}{(\frac{\mu_g}{\mu_w} -1)(\frac{1}{R_1}- \frac{1}{R_2})} $

$\large \frac{f_w}{0.3} = \frac{3/2 -1}{\frac{3/2}{4/3}-1}$

f_{w} = 1.2 m