What causes a rainbow?

Droplets of water in the air act like prisms to refract light: sunlight shines on them and is bent. The sunlight includes the entire visible spectrum, in which each color is represented as a particular wavelength. Each wavelength of light has a different "index of refraction", which means that color gets bent to a particular angle (Serway, 2004). The dispersed light is then seen as the array of colors making the rainbow.

How does the second rainbow occur?

However, according to Serway, a light does not pass through a raindrop in the same way that it does with the prism shown in the picture. Light entering a raindrop is refracted as it enters, reflected off the inside of the opposite wall, and is refracted again as it exits the drop. To cause the secondary, fainter rainbow seen in the top picture, light is reflected twice inside the raindrop instead of once (2004).