We’ve all seen plenty of rainbows, both the natural ones that appear during a rain shower and the artificial ones that appear in the spray of a garden sprinkler, for example. The brighter the sunlight, the brighter the rainbow. Really bright sunlight can cause a second, and even a third, rainbow to appear on the outside of the main one.
When sunlight enters a water droplet, it bounces around so much that it comes out almost the same direction it went in. Because each color of light bends at a different rate, each time the light refracts around the inner curve of the raindrop, the colors separate a little more. The result is a distinctive separation of the colors that always ranges from red on the outer edge, through orange, yellow, green, blue, indigo, and all the way to violet on the inner edge.
Coronas and Glories
These two phenomena are caused by a property of light called diffraction, which causes light to bend around objects.
One of the most fascinating atmospheric optics is called heiligenschein, a German word for "halo." To see a heiligenschein, stand on a dewy lawn on a sunny morning. Turn to face your shadow. If the dew droplets are the right size, you’ll see that your head seems to be ringed by a bright heiligenschein.
Coronas are visible around the moon when moonlight filters through misty clouds. Moonlight bends around the tiny particles, creating a diffuse circle of white light with the moon at its center.
Glories are one of the phenomena that can usually be seen only from an airplane, though a hiker high on a mountain above a cloud layer might see one, too. That’s because glories appear only on the top of a cloud layer.
When an airplane flies above a cloud, sunlight around the shadow of the airplane enters the tiny water droplets that make up the cloud. The light bounces around the inside of the droplet, as it does in a rainbow. But in the case of a glory, the light bends around the edge of the droplet. What results is a brilliant glow of light surrounding the airplane’s shadow.