When direct sunlight strikes falling rain, a rainbow is seen at a point directly opposite the Sun. A double rainbow occurs when some of the light entering the raindrop is refracted into its component colors, reflected off the back interior wall of the drop, and refracted again as it exits the drop. The dark area in between the two rainbows is called Alexander's band.
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Image Courtesy of Carlye Calvin/University Corporation for Atmospheric Research
Rainbows appear in the sky when there is bright
. Sunlight is known as visible or white light and is actually a mixture of colors. Rainbows result from the refraction and reflection of sunlight by these water droplets. As sunlight enters a droplet, it is refracted (it slows and bends), with violet light bending the most and red light the least. Much of the light passes on through the drop, but some strikes the backside of the drop at such an angle that it is reflected back and is again refracted as it leaves the drop and passes to the observer's eyes. Because each light ray bends differently from the rest, each ray emerges at a slightly different angle. The colors are thus separated from each other as they emerge, dispersed into a spectrum of colors from violet to red. It takes myriads of droplets (each refracting and reflecting light back to our eyes at slightly different angles) to produce the brilliant colors of a rainbow.
You can only see a rainbow if the sun is behind you and the rain in front.
You can even make your own rainbow with a garden hose or water sprinkler on a sunny day.
A double rainbow occurs when some of the light entering the raindrop is refracted into its component colors, reflected off the back interior wall of the drop, and refracted again as it exits the drop. In the second rainbow, the colors are reversed where blue is on the outside and red is on the inside. The dark area in between the two rainbows is called Alexander's band.
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