How does rainbow form?

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc, with red on the outside and violet on the inside. More rarely, a double rainbow is seen, which includes a second, fainter arc with colours in the opposite order, that is, with violet on the outside and red on the inside.

Even though a rainbow spans a continuous spectrum of colours, traditionally the full sequence of colours is most commonly cited and remembered as red, orange, yellow, green, blue, indigo and violet. ("Roy G. Biv" and "Richard Of York Gave Battle In Vain" are popular mnemonics.)

Though rainbows are bow-shaped in most cases, there are also phenomena of rainbow-coloured strips in the sky: in the shape of stripes, circles, or even flames. (See Circumhorizontal arc)

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for example : http://www.photocentric.net/rainbow_physics.htm