A convex lens of refractive index 1 . 5 and focal length 18cm in...

From Lens Maker's formula,

For air: $\dfrac{1}{{f}_{a}} = \left(\dfrac{{\mu}_{g}}{{\mu}_{a}} - 1\right)\left(\dfrac{1}{{R}_{1}} - \dfrac{1}{{R}_{2}}\right)$

For water: $\dfrac{1}{{f}_{w}} = \left(\dfrac{{\mu}_{g}}{{\mu}_{w}} - 1\right)\left(\dfrac{1}{{R}_{1}} - \dfrac{1}{{R}_{2}}\right)$

$∴ \dfrac{\dfrac{1}{{f}_{a}}}{\dfrac{1}{{f}_{w}}} = \dfrac{\left(\dfrac{{\mu}_{g}}{{\mu}_{a}} - 1\right)}{\left(\dfrac{{\mu}_{g}}{{\mu}_{w}} - 1\right)}$

$⇒ \dfrac{{f}_{w}}{{f}_{a}} = \dfrac{\left(1 . 5 - 1\right)}{\left(\dfrac{1 . 5 \times 3}{4} - 1\right)} ⇒ {f}_{w} = \dfrac{0 . 5}{0 . 125} \times 18 = 72cm$

Thus, change in focal length of the lens is $72 - 18 = 54cm$