Violet is a spectral color (approximately 420–380nm), of a shorter wavelength than blue, while purple is a combination of red and blue or violet light. The purples are colors that are not spectral colors – purples are extra-spectral colors. In fact, purple was not present on Newton's color wheel (which went directly from violet to red), though it is present on modern ones, between red and violet. There is no such thing as the "wavelength of purple light"; it only exists as a combination.
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 that 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 blue hue as a result of what is known as the Bezold-Brücke shift. The same increase in blueness is not noted in purples.
Violet cannot be reproduced by a Red-Green-Blue (RGB) color system, and must be simulated by a mixture of red and blue (purple). The shade of violet simulated in the color box above is just over halfway between magenta and blue on the color wheel.
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