Friday, July 1, 2011

Friday photo

Rainbow over Hayden Valley. Yellowstone National Park, June 1995.

Rainbows. The only cheaper subject for a nature photographer is sunsets, and that only because they happen to you more frequently. You get especially spoiled living in the mountains, because unlike the Midwest, where the clouds may cover the entire sky for hours on end, a sharp end to the storm is likely to arrive while the rain is still falling. Also, thunderstorms are especially common in late afternoon and when the cloud's edge approaches, the sun is sinking down to the optimal level for creating rainbows (a 42° angle from sun to rainbow to observer).

As far as the mythology of rainbows goes, I'm mostly familiar with the Noah story, where God promises never again to destroy the Earth with a technique that could never have worked in the first place. You can read a quick summary of some alternative mythologies at the relevant Wikipedia article. I often wonder how much anyone took these stories seriously; after all, we all still know the myth of leprechaun's gold at the end of the rainbow, although people who believe it are probably as scarce today leprechauns. Were these stories ever for the adults, or were they always meant as children's entertainment?

Still, I was a bit surprised to learn how long ago people were trying to figure out how rainbows work, from a naturalistic framework. Based only on surviving writings, we can push it back at least as far as Aristotle in the 4th century BCE. It was probably easy enough to understand that the sunlight was being reflected back from water droplets in the air, since the rainbow always appears in the opposite direction from the sun and, in fact, you don't even need rain. The spray from a waterfall will do nicely. But the Greeks were also damned good with geometry, and Aristotle used these skills to explain how logical it was that that the rainbow should have such a perfect semicircular shape. He grasped that the light had to be reflected at a particular angle back to the observer and that the entire set of points that satisfy this condition would form a circle.*

Figuring out why their are colors is pretty tricky, but sometime around 1300 the Persian astronomer Kamāl al-Dīn al-Fārisī used a large, spherical globe filled with water to model a raindrop and worked out that the ray of light is refracted twice, once as it enters the droplet and again as it leaves, having bounced off the far interior wall. He eventually concluded that the white light was somehow being decomposed into the various colors, anticipating Isaac Newton's famous prism experiments in the 1680's

Traditionally, we divide a rainbow into seven colors, but those bands don't really exist and Roy G. Biv is as fictional as Cupid. The bending of the light is seamless and the change in colors ought to appear perfectly seamless as well. But we don't see color that way. We have three types of photoreceptor cone cells in our eyes that each respond to a different wavelength and what we see is a calculation based on those three inputs. Birds, reptiles, and amphibians, on the other hand, have four or five different types of cones and almost certainly can discriminate colors far better than we can. Set your monitor to 256 colors and take off your glasses - that's probably how a bird would feel if you were to suddenly give them human vision.

* The ground cuts off the bottom half of the circle, of course, but full circle rainbows can be seen from the air. I have a cousin who flew on helicopter gunships in Vietnam and tells me that he saw one. I could really envy him the experience, if it weren't for all that war and shooting and danger business that went along with it.

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