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Purple versus violet
The color terms purple and violet cause confusion for many people: they are used interchangeably in some casual conversation. But technically purple is the name of the colour group of many such as violet, mauve, magenta, indigo and lilac. Indigo is a blue-purple, lilac is a light purple and mauve is in between.
Technically, violet is a spectral color (of approximately 420-380nm), shorter wavelength than blue, while purple is a combination of red and blue and is the only color on the color wheel that is not a spectral color (there is no such thing as the "wavelength of purple light": it only exists as a combination). Purple was not present on Newton's color wheel (which went directly from violet to red), but is present on modern ones.
Violet light just varies by wavelength, while purple varies in the proportion of red and blue.
On the CIE xy chromaticity diagram, violet is on the curved edge in the lower left, while purples are the straight line connecting the extreme colors red and violet.
One interesting psychophysical feature of the two colors which can be used to separate them is their appearance with increase of light intensity. Violet, as light intensity increases, appears to take on a far more bluey hue as a result of what is known as the Bezold-Brücke shift. The same increase in blueness is not noted in purples.
http://en.wikipedia.org/wiki/Purple