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Name: Tougharms
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002

Why is a rainbow curved/round?


A rainbow is round because the process is based on angles. Light from the sun hits the little water droplets after a rain. The water droplets act like little prisms. Different colors are sent out at different angles. The sun must be behind you. Imagine a narrow triangle. The narrow angle is at the water drop. One side points toward the sun. One side points toward your eye. The drops for which this device works form a circle. The center of this circle lines up with you and the sun. If the Earth were not in the way, a rainbow would be a complete circle. This is why you can never find "the golden pot at the end of the rainbow". A rainbow is really a complete circle.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Howdy Tougharms!

This one was a "toughie". I had to do a little looking. I had plenty of info on "why" rainbows, but not so much on "why round" rainbows. This is from Hewitt, Suchocki, and Hewitt's Conceptual Physical Science, second edition, p 293. It is published by Addison, Wesley, Longman, copyright1999. (You might try your local library if you wish to learn more; and if you want to see the really good diagrams that Mr. Hewitt draws.) Anyway, here goes:

Why does the light dispersed by the raindrops form a bow? The answer to this involves a bit of geometry. First of all, a rainbow is not the flat two-dimensional arc it appears to be. It appears flat for the same reason a spherical burst of fireworks high in the sky appears as a disk-because of a lack of distance cues. The rainbow you see is actually a three-dimensional cone with the tip (apex) at your eye. Consider a glass cone,the shape of those paper cones you sometimes see at drinking fountains. If you held the tip of such a glass cone against your eye, what would you see? You would see the glass as a circle. Likewise with a rainbow. All the drops that disperse the rainbow's light toward you lie in the shape of a cone-a cone of different layers with drops that deflect red to your eye on the outside, orange beneath the red, yellow beneath the orange, and so on all the way to violet on the inner conical surface. The thicker the region containing the water drops, the thicker conical edge that you look through, and the more vivid the rainbow.

Your cone of vision that intersects the cloud of drops that creates your rainbow is different from that of a person next to you. So when a friend says, "Look at the pretty rainbow," you can reply, "Okay, move aside so I can see it too." Everybody sees his or her own personal rainbow.

I hope that casts "light" on your rainbow query.

Martha Croll

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