Blue

Blue is one of the three primary additive colours; blue light has the shortest wavelength (about 470 nm) of the three primary colours.

A clear sky on a sunny day is coloured blue because of Rayleigh scattering of the light from the Sun. Large amounts of water look blue because red light around 750 nm is absorbed as an overtone of the O-H stretching vibration. Interestingly, heavy water (D₂O) is colourless, because the absorption band is at a longer wavelength (~950 nm).

If your browser supports colour display, the following patch should be blue:

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Usage, symbolism, colloquial expressions

The colloquial expression "blue" is used for melancholy in English speaking countries. See also Blues music. The phrase "from blue blood" is used to mean "from aristocratic background". In German, "blue" means "drunk".

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Blue is also the title of an album by the Canadian singer/songwriter Joni Mitchell.

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