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