“The seven notes of the scale before the return to the octave are analogous to the colors of the rainbow — red, orange, yellow, green, blue and violet, plus the strangely superfluous indigo which made the number up to seven.” — Ian Bostridge
Tenor Ian Bostridge, quoted above, reminds us we are supposed to see seven colors in a rainbow simply because there are seven notes in the normal Western musical scale. Isaac Newton originally agreed with chemist Robert Boyle, who, a few years earlier, claimed to have seen five colors in the spectrum of sunlight split by a prism, that is, red, yellow, green, blue and violet. But Newton, as much mystic as he was scientist, added two more, orange and Bostridge’s “superfluous” indigo, to harmonize with the Western music scale. (In this, Newton was harping back to the ancient Greek Sophists, who saw a connection between colors, musical notes and the seven wandering stars, i.e. five known planets plus the sun and moon.) In his 1704 book Opticks, Newton wrote that the seven colors of the spectrum “may perhaps suggest analogies between harmonies of sounds and of colors.”
So seven it is: ROYGBIV, Richard Of York Gave Battle In Vain. Not to complicate matters, but Newton’s blue is what we call cyan, and his indigo is our blue.
Beyond this, it gets very subjective. Our retinas have three receptors, or cones, for color, responding to red wavelengths (400-500 nanometers), green (450-630) and blue (500-700), giving us the “primary” colors. When two sets of cones are activated equally, our brains tell us we’re seeing the three secondary colors, the same as inkjet printer cartridges: Yellow (red + green), cyan (green + blue) and magenta (blue + red). With all three sets of cones activated, the sky’s the limit. I’ve seen estimates putting the number of colors humans can tell apart in the millions.
How many colors do you see in “nature’s prism,” the rainbow? In the accompanying photo taken recently from Eureka’s boardwalk, I can make out red, orange, yellow,
cyan and violet in the primary bow. In the faint secondary arc above it (note the inverted colors), I think I can see red, orange, yellow and cyan. After that my imagination takes over.
The spectrum of a rainbow is pretty fuzzy compared with one projected through a triangular glass prism. That’s because of the way sunlight is dispersed by raindrops — in a word, messily. Light is first refracted when entering a drop, reflected inside on the back of the drop and re-refracted when leaving it. Red light is refracted less than shorter-wavelength blue light, meaning that we see red on the rainbow’s topside. (The secondary arc, with red at the bottom and blue on top,
results from two additional reflections before leaving.)
The actual geometry of a rainbow is pretty simple. If you’re in a plane looking down, you may be lucky and see a full circle, but because we normally see rainbows from the ground, we see just part of the circle — an arc — centered opposite the sun. The center of the arc would be on an imaginary line drawn from the sun through your eye. In my photo, there should be a pot o’ gold right under the old Table Bluff Lighthouse, which was moved to Woodley Island in 1987.
Barry Evans (he/him, barryevans9@yahoo.com, planethumboldt.substack.com) wants you to know that The New Humbook is on sale at local bookstores and gift shops.
This article appears in On the Matter of Censure.