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. They take the form of a multicoloured  arc, with red on the outer part of the arch and violet on the inner section of the arch.
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A rainbow spans a continuous spectrum of colours; the distinct bands are an artifact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.