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Rainbow Colors
Fri Dec 10, 2004
Listen in RealAudio 
So how many colors are there in the rainbow, anyway? If you know the phrase
"Roy-G-Biv," than you know the seven colors we're supposed to see. Hi, I’m Bryan
Yeaton, and this is The Weather Notebook.
ROY G. BIV is an acronym for those seven hues. In order, from top to bottom, they're
red, orange, yellow, green, blue, indigo, and violet. But the ancients saw rainbow
colors in a much different way. Over two thousand years ago, the Greek philosopher
Aristotle proclaimed the rainbow had just three colors: red, green, and blue. A
millennium later, Arabic scholars saw four colors: red, yellow, green, and blue, one for
each of the four elements and the four seasons.
It wasn't until the 1600s that Isaac Newton, who explained how a raindrop separates
white light into colors, declared that the rainbow had seven hues. But his rationale
may have had as much to do with hearing as with seeing. As every "Sound of Music"
fan knows, a musical scale has seven natural tones: do-re-mi-fa-so-la, and ti.
Apparently Newton applied this same "rule of sevens" to the rainbow.
Ocular anatomy gives us yet another story. Our eyes have color photoreceptors cells
called cones. There are three different kinds: each structured to see a different part of
the light spectrum. Our brain blends the input from the three cones to give a complete
spectrum picture. But the color-sensitive cones need lots of light to function. When the
sun goes down, your other photoreceptor cells—the rods—take over. The rods see
only in shades of black and white. That’s why, when you’re out walking at dusk, the
colors all fade into an arrangement in gray and black.
The Weather Notebook is produced by the Mount Washington Observatory, with
funding from the National Science Foundation and Subaru: Driven By What’s Inside.
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