The rays of light converge when they pass through the drops of water. Thus, the drop of water behaves like a convex lens.
A water drop in the air has a higher refractive index as compared to air. And because of its convex shape, it tends to converge a parallel beam of light falling onto it, and therefore it acts as a converging lens. A convex lens is a converging lens. When parallel rays of light pass through a convex lens, the refracted rays converge at one point called the principal focus.