Question

A spherical mirror and a lens are immersed in water. Compare to the way they work in air, Which one do you expect will be more affected by water?

Answer 1

Explanation about the spherical mirror

1. The focus of spherical mirrors does not depend on the refractive index.
2. Hence, when a spherical mirror is immersed in water then the focus of the mirror is unchanged.

Explanation about the lens

1. The lens maker formula is $\frac{1}{f}=\left(\frac{{\mu }_L}{{\mu }_M}-1\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$, where ${\mu }_L=$refractive index of lens material,${\mu }_M=$refractive index of the surrounding medium, $R_1=$Radius of curvature of $1^{st}$refracting surface,$R_2=$Radius of curvature of $2^{nd}$refracting surface.
2. The focus of a lens depends on the refractive index of the surrounding medium. So, when it is immersed in water, its focal length increases.

Hence, the lens will be more affected by water than a spherical mirror, as the focal length of the lens increases when it is immersed in water whereas there is no change in the focal length of a spherical mirror.