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Not all colored arcs or rings are rainbows, but rainbows might be the most
recognizable. There is something visually irresistible about the transformation of
everyday sunlight into previously hidden colors. One of my hopes with this book is that more people will begin to recognize the existence and uniqueness of other colored arcs - coronas, glories, halos - as well as rainbows. However, this chapter is all about the beautiful phenomenon we call the rainbow.
As its name implies, a rainbow is a bow in the sky that is formed when light interacts with raindrops. Actually, water drops from any source will suffice, whether rain, a garden hose, or a fountain. Figure 4.1 illustrates the bright colors a rainbow can produce under the right conditions. Figure 4.2 reminds us of the beauty and impressive size of a double rainbow, and Fig. 4.3 gives a closer look at a double rainbow to emphasize the reversal of colors between the primary and secondary bows. Notice also how dark the sky is between the bows, a region called Alexander’s dark band (an angular region from where light is effectively removed to create the rainbow).
Much of what we see in a rainbow can be explained with geometric optical ray tracing, although the details require more complicated theories. The basic explanation relies on refraction (bending) and reflection of light inside rain drops. The simple explanation relies on fictionally spherical drops; real ones (especially large drops) are shaped more like pillows than spheres but never like the tear drop so widely used to denote rain in popular media. Nevertheless, valuable insight can be gained with a simple, spherical rain-drop model.
Rainbows are colored because light entering and leaving a rain drop refracts, or bends, by an amount that depends on the wavelength. This is called dispersion - the same mechanism that produces colors from a prism. For water, glass, and many optical materials, blue light refracts more strongly than red light. As a convenient pair, we often discuss blue and red light with the understanding that a continuous spectrum of colors actually exists within white light. However, the color purity in rainbows is reduced well below 100% by competing background light and other factors, although the rainbow still presents the purest colors of all atmospheric optical phenomena other than a reddened sun.
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