Raindrops are made up of water, so when light passes from air to water, that is, from rarer to denser medium, it gets refracted. Thus refraction is a natural phenomenon. 

However, the angle of refraction sometimes is greaer than the critical angle for air-water interface. In that case, no refraction happens. But another phenomeno called total internal reflection takes place, in which all the light entering the raindrop gets internally reflected. Under this condition, we see raindrops reflecting light rays.