Many optical phenomena that appear in the sky may be explained using the geometrical ray tracing described in this book. A few that will be discussed here are rainbows, halos, sun dogs, and the optical illusion of mirages in the desert.

Rainbows

A rainbow is an optical phenomenon that causes a nearly continuous spectrum of light to appear in the sky. It is only present when the rain location is opposite the sun's direction from the observer – in fact, when they are 180° apart. As white light from the sun shines onto drops of water, a polychromatic bow is formed, with red in the outermost arc and blue/violet on the inside of the bow. The infrared color regions beyond the red are ignored; however, they do exist. This is the most traditional rainbow, in which sunlight is spread out into its spectrum of colors and diverted to the eye of the observer by spherical water drops. The “bow” part of the word comes from the fact that rainbows contain a group of nearly circular arcs of color (blue to red), all having a common center of curvature. A second rainbow sometimes appears as a fainter arc, with colors in the opposite order, i.e., with violet on the outside, and red on the inside.