Re: Why isn't indigo considered a color of the visible spectrum anymore?

The labels we apply to colors are arbitrary. We could just as easily say that the colors of a white-light spectrum are scarlet, tangerine, lemon, lime, sky, navy, and grape. The fact that some person or group has dropped indigo from the "official" list of colors does not mean that we can not continue to use "ROYGBIV" to describe the spectrum. I, and a bunch of other optical physicists, still consider indigo to be a color of the spectrum.

There are lots and lots of pages on the Web dealing with color and spectra. Using AltaVista I found, among a bunch of other sites, this Web page about colors which does use the color indigo.

On televisions and computer monitors the spectrum is constructed using only three colors. Although the usual colors used as primary colors, to produce the sensation to our eyes of all the other colors, are blue, red, and green, there is no particular reason why any three widely spaced colors could not be used to produce the entire spectrum to our eyes. (You can find lots of previous answers in our archives using our search engine about primary colors by simply searching using those words.) Because red, blue, and green are so commonly used, we sort of consider "the primary colors of light" to be red, blue, and green, and if these three are mixed in equal proportions two at a time we obtain magenta, cyan, and yellow, so it is common these days to consider the "colors of the rainbow" to be red, yellow, green, cyan, blue, and magenta. I suspect that indigo has been "dropped" from some lists because it introduces a seventh "color", and it seems to be easier to explain only six colors.

But "RYGCBM" doesn't look very much like "ROYGBIV"!!!!! You can use "ROYGBIV" and feel perfectly right in doing so, but if you would like to come up with a sentence that produces "RYGCBM" go right ahead. Maybe you will become famous as the person who originated the sentence.

In actual fact our eyes can distinguish tens of thousands of colors. But to keep things simple we say that there are only six or seven colors, just to make it easy. Can you imagine having to name tens of thousands of colors?? The makers of crayons are pretty creative in naming colors, though. But a "ROYGBIV" equivalent for tens of thousands of colors would be impractically long!

Happy spectrums!!!!

John Link, MadSci Physicist