What phenomenon explains why a rainbow is formed?

The formation of a rainbow is primarily explained by the dispersion of light. When sunlight enters a raindrop, it is not only refracted, or bent, but the different wavelengths of light (colors) are also spread out due to their varying degrees of bending in the water. This process separates the light into its component colors, creating the visual spectrum of a rainbow.

Dispersion occurs because the refractive index of water varies with wavelength; shorter wavelengths (like blue and violet) bend more than longer wavelengths (like red). As light exits the raindrop, this separation of colors results in the beautiful arc of a rainbow, with red typically on the outside and violet on the inside.

While reflection involves light bouncing off a surface, and refraction refers to the bending of light as it passes from one medium to another, these processes alone do not account for the visible spectrum that we identify as a rainbow. Diffraction, on the other hand, involves the bending of light waves around obstacles and does not play a significant role in the formation of a rainbow.