first you should know what a rainbow is -

A rainbow is an optical and meteorological phenomenon that is caused by reflection of 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 multicolored arc.
Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.


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 while 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 colors reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.

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