Demote Indigo

I’ve been thinking about the rainbow, and it’s not clear to me that indigo should have its own band. It should be demoted, much like Pluto was demoted from being a planet. Don’t get me wrong, I’m not saying indigo isn’t a color in the rainbow, or that The Indigo Girls shouldn’t have a band. But teal is also a color in the rainbow, and no one is lobbying for teal to have its own band.

Indigo is pretty much just dark blue, it’s neither special nor distinct when you look at a rainbow.* I’m pretty sure that indigo is only a band because the people who decided these things liked the number 7. In addition to being a lame band, removing indigo gives a rational symmetry to the rainbow. With 6 bands, the rainbow is in alignment with introductory art classes, where we learn of three primary colors (red, yellow, blue) and three complementary colors (green, violet, orange). Remove indigo from the rainbow, badda bing, badda boom, Nature, art and physics are one. I know that ROY G BIV is an iconic acronym, so in order to remember the new system I recommend the expression, “ROY G BIV, but I am demoted.”

I’m just here to help.

While I’m on the topic of colors and rainbows, I would also like to advocate using “violet” only to refer to the short wavelengths in the rainbow, and to reserve “purple” for combinations of red and blue. Admittedly, this probably only bothers me. Still, it’s my blog. At least I think this is a blog. In any case, although our eyes cannot distinguish purple from violet, they are fundamentally different in terms of energy spectrum and would behave differently, say, when passed through a prism. Even a trained eye cannot discern the difference between combinations of wavelengths of light and a single wavelength. This is in contrast to the trained ear which can discern the different notes within a chord.

The similarity between purple and violet fascinated 19th century metaphysicists. They were hot on analogies and statements like, “As above, so below.” Here’s how it came up: the entire spectrum of visible light (the light in the rainbow) goes from wavelengths of about 390 nanometers (nm) (violet) to about 700 nm (red). Thus, the entire visible spectrum is slightly smaller than a doubling of frequencies/wavelengths. In musical terms, this would be just shy of an octave. To a metaphysicist, this meant that just after violet, another red was coming, perhaps a pastel, I don’t know, but another red. It made sense of why violet could be mimicked by red and blue. In the metaphysical theory, violet is a transitional “mix” of red and blue, except the red involved is the next octave above the one we see. I always liked this idea, although it makes little physical or physiological sense. The facts that visible light covers a frequency range slightly smaller than an octave and that violet looks like blue plus red are seductive. However, as I used to continually remind graduate students, “A lot of things make sense, but only a few things are true.” It’s why we do experiments rather than see who wins a debate about what ought to be.

*I am compelled here to assure the disgruntled reader that I am fully aware that the rainbow represents a continuous spectrum of wavelengths and the bands we see are subjective in nature, dependent on the mechanistic vagaries of the pigments in our eyes and our cognitive architecture. I’m not addressing this at the moment because it is obvious and boring, unless you didn’t know that, in which case it’s pretty fascinating. Either way, we perceive bands, so I get to talk about them.