That Accursed Rainbow Song

No, not that one; that one's fine. And this version's lovely, of course. I'm talking about the rainbow song that my kid sings in preschool to the tune of Twinkle, Twinkle, Little Star:

Red and orange, green and blue,
Shiny yellow, purple too.
All the colors that we know
Live up in the rainbow.

Let's pass over the metric stumble in the final line. Let's talk about scientific stumbles instead.

Purple is not in the rainbow. "Purple" is what happens when you mix red and blue, and that can't happen in a rainbow because red and blue are on opposite sides. When you break up white light with water or a prism, the colors are organized by wavelength, and while red is the longest wavelength of visible light, blue is on the short end.

In a rainbow, we get to see orange between red and yellow, and we get to see green between yellow and blue. And on the far side of blue, we get to see violet--the shortest wavelength human eyes can detect.

At this point (if not much earlier), some people will probably roll their eyes and state: "You say violet, I say purple; let's call the whole thing off." I'll simply answer that with links to Wikipedia and Quora, so I can go on to the other aspect of the rainbow song that drives me bananas: the colors are out of order.

Every time my daughter draws a rainbow--which is at least twice a day--she sings the song to remind herself how to do it. Thus, as a direct result of the ditty's failure to respect the facts of electromagnetic radiation, our house is full of rainbows that look like this:

Was this entire post an excuse to post my kid's artwork online? WAS IT? I really couldn't say.

Now, I am wholly in favor of free artistic expression. We've been reading Eric Carle's The Artist Who Painted A Blue Horse and I would never in a million years tell my kid she drew something "wrong." Still, would there be any harm in amending the song to reflect reality? Hey, maybe I can fix the meter while I'm at it.

Red and orange, yellow and green,
Blue and violet, that we've seen,
All the colors in white light--
A rainbow's such a lovely sight.

I hereby release these lyrics into the public domain. Go ahead and use them in all the schools and playgroups. I ask no royalties, no attribution--only the dissemination of accurate knowledge about optics to our impressionable youth.


N.B. "What about indigo?" demands the righteously indignant reader. "If you're going to be pedantic about purple and violet, how dare you leave one of Newton's seven canonical colors out of your revisionist lyrics?" 

Dear Righteous Reader, at first this troubled me too, but it didn't trouble me quite as much as the nagging feeling that I've never been able to see indigo in the rainbow anyway. 

A bit of research revealed that I'm not the only one. There's a nice 1972 paper in the American Journal of Physics called "Why Did Newton See Indigo in the Spectrum?" and the argument against indigo is summarized cogently in this 2015 article at the National Post.