Why do we see rainbows?

Rainbows occur due to the reflection of light from water vapor that acts as a prism. In order to see a rainbow, the observer must be in front of the sun while the water vapor or rain is in front of the observer.

Sunlight strikes the water vapor at a low angle to create the prism effect. Water distorts the wavelengths in the sunlight. People see red, violet, blue and orange among other primary colors. Multiple reflections of the light inside the same droplet of water creates an appearance of a double rainbow with a layer of colors in the opposite arrangement of the first rainbow.