Thick lens

A thick lens consisting of two spherical surfaces is centered on an optical axis. The radius of curvature of the left interface is $R_1$ and the radius of curvature of the right interface is $R_2$. The thickness of the lens measured along the optical axis is $d$. Light rays leave an object $o$ to the left of the lens and are refracted at both interfaces. The refraction can be calculated using Snell's law of refraction at a spherical interface. The index of refraction is wavelength dependent. In this app you can specify the index of refraction for red, green, and blue light in the lens and in the environment that the lens is immersed in. If the lens is in air, $n_{env}=1$. The different colors are bent through different angles. This is called chromatic aberration. If the index of refraction is made the same for all three colors then it is still not always possible to focus the light to one point. This is called spherical aberration. It is possible to focus the light by only using rays that make a small angle with the optical axis.

Water has an index of refraction of $n=1.33$. A round droplet will act like a lens.