Blue

Blue light is one of the three primary additive colours, and has the shortest wavelength of the three primary colours.

Blue light has a wavelength of roughly 470 nm.

A clear sky on a sunny day is coloured blue because of Rayleigh scattering of the rays of 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.

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

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