Regular scattering, caused by refraction and chromatic dispersion, is responsible for the colours seen in rainbows.
Refraction occurs when light changes speed and direction as it passes from one transparent medium (like air) to another (like water).
Chromatic dispersion is the phenomenon where light separates into its various colours, each with a slightly different wavelength, which bend at slightly different angles during refraction.
Scattering in raindrops obeys the laws of both reflection and refraction, commonly referred to as Snell’s law. Here are three related descriptions of what causes scattering when visible light strikes a raindrop:
When light of a specific wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of water and air.
When rays of light of a single wavelength strike a raindrop at different points, scattering is primarily determined by the angles at which they enter the droplet.
The interaction between refraction and chromatic dispersion gives rise to the appearance of rainbow colours when parallel white light rays strike various points on the surface of a raindrop.