• Light can undergo a second reflection within a raindrop, and then exit in a completely different direction. If the light enters the raindrop at the right location, the ray that exits after two reflections will be directed toward the ground. The angle away from the sun is somewhat different from that of the primary rainbow; it appears higher in the sky. This is the nature of a double rainbow.
• Visible light isn’t special in terms of forming a rainbow. Infrared radiation does the same thing, for example, but our eyes can’t see it. However, infrared-sensitive cameras have indeed taken pictures of the “infrared rainbow.”
• Raindrops are not the only things that can cause sunlight to be refracted and/or dispersed. Ice crystals in the upper atmosphere can do the same thing. Because the crystals are not spherical (but do have regular, well-defined shapes) the light comes back to us at angles different from that of the rainbow. These are sometimes seen as “sundogs”—colorful arcs of light seen about 22 degrees from the sun, generally on cold sunny days.
• The ice crystals responsible for sundogs can also form a “halo” around the sun at a radius of 22 degrees. But the sun is much too bright for this to be seen unless it is blocked out by some object along the line of sight. Thus, it isn’t a good idea to look for this (and it is important to never look close to the direction of the sun; it is so bright that it is dangerous to our eyes!). However, this halo can often be seen around the full moon on cold winter nights.
• The wavelength of radio waves that carry the music to your car radio is about 3 m (about 10 ft). You can actually notice this at times. Have you ever had the radio station fade out a bit when you stop your car? Try rolling forward (or backward) a couple of feet. Often the reception improves. This is the result of what we call interference. You can move from a region of “destructive interference,” where the signal is bad, to one of “constructive interference,” where the signal is good, by moving a distance equal to half of the wavelength.
• Scientists use special instruments that disperse the spectrum of stars and other objects that they view through telescopes. The spectrum tells them a lot about the temperature and structure of these objects.
