# Six Words: Rainbow!

How many colors in a rainbow? Six!

What? Doesn’t “everybody know” there are seven?

No, but seven is what most of us were taught: red, orange, yellow, green, blue, indigo, and violet.

That’s how I learned them in school. We even were taught to remember them using the name ROY G BIV.

Too bad it isn’t really true. Like so many other things we learned in school.

It’s Isaac Newton’s fault. Newton freely admitted – even emphasized – that he didn’t have much of an eye for distinguishing one color from another.

Despite this handicap, it was his famous prism experiments that gave us the so-called “colors of the rainbow” and of what we now call the color wheel or color circle.

His first pass gave the rainbow just five colors – red, yellow, green, blue, and violet.

This didn’t satisfy him. Newton was taken – okay, obsessed – by the notion that a rainbow should have seven colors, corresponding to the seven major notes of the musical scale (do, re, mi, fa, so, la, ti).

### Twisting Facts to Fit

To get this result, he stuck in orange (between red and yellow) and indigo (between blue and violet). Hence the sequence we now remember using the ROY G BIV mnemonic.

Perhaps this arrangement isn’t strictly “wrong”. After all, the colors – whether in a rainbow or a wheel – are infinite in number. We can break them down into arbitrary regions and label these any way we like.

The problem with Newton’s seven-color scheme is that it isn’t balanced or symmetrical. The easiest way to see this is by dividing the spectrum into primary and intermediate colors.

### Why We Need Balance

Our primary colors are red, yellow, and blue. For discussion, let’s lay these out in a circle (as in a color wheel). Then let’s fill in blank spaces with intermediate (secondary) colors.

Orange goes between red and yellow. Green goes between yellow and blue. Purple (or violet, if you prefer) goes between blue and red. This bring us full circle.

That’s six colors. Where in this scheme does indigo fit? The only logical place is between blue and violet (purple). But this places two intermediates (indigo and violet) between the red/blue primary pair. The other pairs (red/yellow and yellow/blue) have only one intermediate apiece.

This clearly is out of whack. As noted, we can divide up the color spectrum in any arbitrary way we like. But we need to do so in a way that’s consistent.

If we designate three colors as primary, then we need three intermediate colors as secondary. Three, no more or less. Six in all.

I suppose we would pick any five equidistant same-size regions on the color wheel, give them names, and call these primary colors. But then we’d need also five intermediates.

Or we could decide that between each of our primaries (however many we allow) we’ll have three intermediates. Or any other number. We can name these as we like. But if we have three (or 10, or 50) between any primary pair, we need the same number between any other primary pair.

No matter how we slice-and-dice the color spectrum, it’s hard to deny that Newton stuck in indigo where it isn’t needed. And where it isn’t needed, it doesn’t belong.

Thus the basic rainbow (or color wheel) should have six members: red, orange, yellow, green, blue, and purple.

Okay, violet.

But forget indigo!

And I’m not sure what we’ll do for a memory aid to replace ROY G BIV. Somehow “ROY GBP” hardly seems adequate.

#### Six Words: Rainbow! — 10 Comments

1. Well, well… growing up in India I learned VIBGYOR instead of ROYGBIV. I didn’t at first get ROYGBIV when I saw it once in a Google+ photo group. But then when I saw the colors in the photos being posted I figured it out and then it struck me: this is upside down! I brushed it off as yet another thing that Americans have inverted just to be different (on-off for light switches, the direction a key goes in the lock, left-right side of the road for driving.. &etc. haha)
My daughter likes a rainbow song from the cartoon show Peppah Pig (who is British, BTW) and that song ignores indigo. In fact, I have seen Barney (the purple dinosaur) sing a rainbow song without indigo as well. I have never understood indigo. It sounds like a made up color. The only time I’ve seen indigo is in a clothes whitener. I know it’s strange. I think the dye is in low dilution so it doesn’t actually make the white cloth indigo but probably helps the upper wavelengths, and ultravioltets bounce off better making whites appear whiter. Speaking of which indigo comes from the indigo plant and is used a dye. But the indigo dye appears to me a lot more like a darker blue than a blue-violet. It’s all very murky anyway…

While we are on the topic, look up “indigo revolt”. It’s an uprising of indigo farmers in India, in Bengal to be precise and started in a place I personally know: where my mother grew up.

• Let me get this straight: They use a dark-blue dye (indigo) to make clothes … whiter! Now I know what I’ve long suspected — that reality has jumped its track and gone completely off-kilter.

2. How many colors in Noah’s rainbow?
We saw 4 in one day in the mountains.
Indigo dye was replaced by synthetic dye by BASF.

• Noah’s rainbow — probably the same colors as ours. Keeping in mind of course that Noah’s rainbow was a symbol of God’s eternal promise to humanity. If we then interpret the colors as races and tribes of of humanity, or their sequence as ages of history, perhaps we’d have to vary them?

Remember that the breakdowns are arbitrary. They’re matters of definition. I’m just saying our framework, whatever it is, needs to be consistent with itself.

Four rainbows in one day is a lot! Mountains are the best place to see such a thing: You can see up, down, and in a bunch of directions. Thanks for sharing that exquisite memory.

3. Even infinite is technically wrong. There are a limited number of molecules — far into the trillions. probably exceeding grains of sand on planet earth — but still limited.

• Good catch, John. You’re right: The number of molecules in the whole universe is finite. Heck, even the number of sub-atomic particles is finite (though very large).

It’s also my impression that light itself exists in what physicists call “quantum states”. That is, each wavelength is unique and sharply distinct from every other wavelength. They go in steps or breaks, not a continuous flow. If that’s true, then the number of theoretically possible colors also is limited. (Because each wavelength corresponds to one color.)

To put it another way, there are adjacent wavelengths (colors) such that it isn’t possible to have a wavelength halfway between them. I’m not sure whether this is accurate. My physicist friend Jonah Miller would know. Jonah, are you out there? What say ye?

• Thanks, Grace. Although as John notes in another comment, my statement about infinite colors wasn’t strictly accurate.

Perhaps I should have said “almost infinite”. But that doesn’t seem logical, either. Next to infinity, any two finite quantities look pretty much the same — infinitesimal. ≧◔◡◔≦ It’s all a question of perspective.