Submodule 4: Secondary Rainbows
Steve Beeson, Arizona State University

What is a secondary rainbow?

Every once in a long while, if you get lucky, you may see a second rainbow on the outside of the first, brighter rainbow. This is the "secondary rainbow" and occurs because of raindrops that are higher in the atmosphere than those causing the "primary rainbow".

The higher raindrops will internally reflect the sunlight twice rather than just once.

Notice that the sunlight hits the bottom of the raindrop to make the secondary rainbow. There is still sunlight hitting the other parts of the raindrop. However, some of the light is simply transmitted through the raindrop without reflecting, and some light is reflected and refracted, but not into our eyes. That light is for someone else's rainbow.

Notice also that the red and blue lines are switched in comparison with the colors from the raindrops making the primary rainbow.
What does this mean in regards to the colors of the secondary rainbow?
What color is the innermost "bow"?
What color is the outermost "bow"?

We find that the secondary, outer rainbow goes from red on the inside to violet on the outside.

Return to Rainbows

"Why is the sky inside the rainbow brighter in the picture above?"

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Light & Optics
Submodule 4
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Copyright &copy1995
Steve Beeson, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287