1, the glass tube with water droplets is a convex lens. A convex lens refers to a mirror with a convex center and a thin side. A concave lens is just the opposite of a mirror with a convex surface in the middle.

2. Raindrops are round under the action of surface tension. Because of gravity, they are not round, but simply protrude from the middle, which is equivalent to the thick middle of the convex lens and the thin edge. The refraction of water to light makes raindrops become convex lenses, and from its effect on light, raindrops have a convergent effect on light.