Question

A thin lens of glass is immersed in water .What will be the effect on the power of the lens

Answer 1

Let the focal length of lens in water be $f_{w}$
and we also assume $n_1$ is the refractive index of water and $n_2$ is the refractive index of lens .

according to lens maker formula,
$\frac{1}{f_w}=\left[\frac{n_2}{n_1}-1\right]\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$

now you should notice that refractive index of glass is greater than refractive index of water. e.g., refractive index of glass, $n_2$=1.5
and refractive index of water, $n_1$=1.33
so, $\left[\frac{n_2}{n_1}-1\right]\approx0.28$

while when lens in air :
$n_1=1$ and $n_2=1.5$
then, $\left[\frac{n_2}{n_1}-1\right]=0.5$

here we observe after immerging of lens in water focal length of lens decreases. so, power of lens increase [ because power is inversely proportional to focal length ]