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A rainbow is an optical and meteorological phenomenon that is caused by both reflection and refractionof light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

All rainbows are full circles, however, the average observer only sees approximately the upper half of the arc, the illuminated droplets above the horizon from the observer's line of sight.^[1]

In a "primary rainbow", the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted (bent) when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets. http://en.wikipedia.org/wiki/Rainbow

In Genesis 9:13 God says He has put a rainbow in the sky to show He will not send another flood like Noah's Flood which covered the world and caused the deaths of all animals and humans except those on the Ark.