Wikipedia: 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).

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Usage, symbolism, colloquial expressions

The colloquial expression "blue" is used to describe melancholy or sadness in English speaking countries. See also Blues music.
The word "blue" is often used in reference to the Police force; eg. "Boys in Blue", "The blue line"
The phrase "of blue blood" is used to mean "from an aristocratic background". This expression derives from the bluish appearance of people suffering from hemophilia, which was a comparatively common condition in European nobility in the 16-19th centuries.
A "Blue movie" is a slang term for a pornographic film. There are also "Blue magazines." this term is more common in Great Britain than the United States.
Users of Microsoft Windows often use the term "blue" to describe a computer that has encountered a Blue Screen of Death.
"Blue laws" is a slang term for laws regulating issues of morality, such as alcohol, gambling, or sexually-explicit materials.
In Australia, a "blue" can also describe a fight or an argument.
The German word for blue is used for "drunk".
In Japanese, the word for blue is used for "young".

Blue is also the title of an album by the Canadian singer/songwriter Joni Mitchell. Kind of Blue is the title of an album by Miles Davis, which has become one of the biggest selling jazz recordings in history.

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