Indigo missing

rainbow colorsEveryone knows the song…

Red and Yellow and Pink and Green, Orange and Purple, and Blue…

Not exactly the best mnemonic for remembering the colours of the rainbow, that purple should be “violet” after all, and then there’s the little problem of “pink”!

Much better is VIBGYOR (violet, indigo, blue, green, yellow, orange, and red) as cited in Newton’s famous prismatic experiments. But, there’s a problem even with that list. Where is this indigo he mentioned? Can anyone really distinguish between violet and blue? To my eye, there certainly isn’t a jeans coloured slice in the spectrum, and as chemist M. Farooq of the University of Karachi in Pakistan suggests (on the CHEMED-L list this week) this isn’t due to limitations of the mirror-water or the glass being used to produce the rainbow.

An article in the American Journal of Physics, he points out, drew attention to the fact that indigo does not exist in the spectrum some years ago and that instead was nothing more than one of Newton’s “preconceived notions”. According to the AJP paper, Newton adapted the colours of the artist’s wheel – red, orange, yellow, green, blue, violet, and purple.

The indigo of our mnemonic is actually violet, and what Newton referred to as violet is probably what we call purple. Of course, purple is not present in sunlight but is a colour of mixed pigments on the artist’s wheel. The electromagnetic spectrum isn’t realy divided into component colours in this way at all, anyay, it’s a continuous spread of light wavelengths, at least down to the quantum level and ignoring spectral lines.

The US National Bureau of Standards correlates particular wavelengths with a colour, nevertheless:

400-465 nm violet
465-482 nm blue
482-487 nm greenish blue

597-617 nm reddish orange
617-780 nm red

It’s all very well laying down the colourful law like that, but your idea of “reddish” might be slightly different from mine, in fact I might see orangey-red when you perceive reddish-orange (maybe it’s another example of the ambiguity in art I discussed recently in this blog). Moreover, as John Denker points out, there is a “band” between yellow and green that if the word chartreuse is in your vocabulary you might label it as such. “The question is not whether the band is there, but whether the observer chooses to take notice of it,” he says. “This whole colour-naming issue depends relatively more on cultural and behavioral factors, and depends relatively little on physics,” he adds.

On the same discussion group Thomas O’Haver of the University of Maryland asks, “Is there really in value in having students memorize something like this?”

It still doesn’t help much with the words of that song, though.

Ref: George Biernson “Why Did Newton see Indigo in the Spectrum”, AJP, 1972, 20, 526

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10 thoughts on “Indigo missing”

  1. Huh? That’s the point I’m making indigo was fabricated by Newton so that there would be the mystical number 7, but not everybody knows that at all. Most visible spectrum mnemonics claim red orange yellow green blue indigo violet. Not purple by the way, violet…

  2. indigo is not part of the rainbow
    red orange yellow green blue and purple everybody knows that

  3. V good thanks I’ve been going mad with the definitions for both indigo and violet llately and now i realize this is okay! pinky blue and purple will be fine then. -painting rainbows in murals soon!

  4. A good way I learned to remember the colours was “Richard Of York Gave Battle In Vain” which should perhaps now be “Richard Of York Gave Battle Vainly”.

  5. I realized the other day that one shouldn’t need a mnemonic in the first place if one is at all familiar with color to begin with — e.g., of course orange falls betwen red and yellow.

  6. Interesting information George. And, I suppose whether or indigo exists is quite subjective. Scientifically, in one sense, it most certainly does not. NIST defines 400-465 nm as violet and 465-482 nm as blue. Nothing in between.


  7. The color does exist, wavelength 435 nm between blue and violet. With its namesake in Indigo dye it would seem that at one point humanity chose to distinguish between the color of the flowers violet and indigo.

    The color chart in the Wikipedia article shows the colors blue, violet, and indigo and there is definitely a discernible difference between them. Interestingly enough, indigo was also the color for the untouchable caste in India so discrimination of the color in all possible senses of the word was obvious.

    From Wikipedia: One can see spectral indigo by looking at the reflection of a fluorescent tube in a non-recordable compact disc. This works because the CD functions as a diffraction grating, and a fluorescent lamp generally has a peak at 435.833 nm (from mercury), as is visible on the fluorescent lamp spectrum. Also from Wikipedia: “It is impossible to represent spectrum indigo exactly on a computer screen, because true spectrum indigo is outside the color triangle or gamut of the RGB color space defined by the monitor primaries.” This could lead one to question whether we are discarding indigo because it does not reproduce well in our chosen approximations to reality or whether Newton and the population of India are wrong in seeing a color others deny.

    As usual, your articles lead one on interesting paths of thought.

    George Girod by email

  8. Yeah, but that’s the point, there is *no* indigo, according to various studies…so you’re left with ROY G. BI. My mother trained as an eye nurse and the mnemonic she and her colleagues were taught gave the colours in reverse to what one usually sees – VIB GYOR – it never struck me as much of a mnemonic, but I must say I haven’t forgotten it. Of course, it will have to be revised now to IB GYOR.


  9. I am not sure where the song came from but ROY G. BIV is a great way to remember the colors. Red Orange Yellow Green Blue Indigo Violet

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