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How do rainbows form?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. Light is made up of a spectrum of colors, each with a different wavelength. When sunlight passes through raindrops, the drops act as tiny prisms. Prisms refract light and break it into the colors of the spectrum. The light is reflected off each drop's side as it enters, is broken up into a spectrum, and bent as it exits. Each wavelength of light bends at a different angle. When the light exits the drop, it is traveling at an entirely different angle than it was before, all of its component colors having dispersed into separate beams. Red bends at 42 degrees and violet bends at 40 degrees, for example. The different angles cause the colors to spread out across the sky in a circular rim we know as a rainbow.

    Light doesn't travel in a straight line, so it will continue bending and refracting as it moves through the air. Rainbows look different, depending on where they're viewed from. Because most are viewed from the horizon, it appears that the rainbow simply forms an arc that stops at the horizon; however, a rainbow viewed from above will in fact be visible as a full circle.

    Of course, all of this bending of light in water drops only comes together to make a beautiful rainbow if the right weather is present and the viewer is in the right location in relation to it. To see a rainbow, you have to be standing with the sun behind you and the rain and rainbow opposite you and the sun [source: University Corporation for Atmospheric Research]. It also might take nothing more than a fountain or other producer of mist to create a tiny rainbow in just the right situation.

    In fact, to see the action of rainbows forming and bending on a less grand scale -- and without waiting around for Mother Nature to cooperate -- you can create the effect with a few household items on a sunny day. Using a clear glass filled with water, take the glass and a piece of white paper to a sunny location in the house, usually near a window. Hold the glass of water above the paper just so until you see the light pass through the glass of water, bending and forming colors on the sheet of paper. Moving the glass to different heights or angles in relation to the paper should create different effects [source: Science Kids].

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