When is a Bird Feather like a Blue Sky?
A recent item by syndicated columnist L.M. Boyd read like this: "No bird is really blue. None produces blue pigment. What you see when you see a blue bird is the light that bounces off the bird's true pigment. It absorbs all the rays except blue." .
I thought about this for a while and decided that I didn't believe it. After all, I reasoned, whether the light which we identify as a certain color is transmitted or reflected, doesn't what we perceive it to be define what color the object is?
To check Boyd's claim, I called upon the family's blue parakeet to sacrifice a couple of his feathers for the sake of science. I was totally unprepared for what I discovered. Sure enough, if I held the feathers up to the window so that I was viewing them by transmitted light, they appeared to be a pale yellowish color. Then, as I slowly turned around to put my back to the window so that I could look at the feathers by reflected light, they gradually changed color until they were deep blue by the time the sun was falling over my shoulder.
Suitably chastened, I remembered a question that readers of this column regularly ask: "Why is the sky blue?" Glenn Shaw and Neil Davis both addressed this question in earlier columns, but now seems to be an appropriate time to recapitulate.
The visible spectrum ranges from light with short wavelengths of just longer than ultraviolet (violet and blue), through longer and longer wavelengths of green, yellow, orange, and red, with infrared just beyond the longest visible wavelengths. Light of short wavelengths has a tougher time making it through the atmosphere (which is fortunate for us, or we would all soon be suffering lethal sunburns from the ultraviolet). The sky is blue because tiny air molecules reflect blue light more effectively than the other colors, "scattering" it by reflecting it back and forth between molecules. Light of the longer wavelengths makes it through the atmosphere more easily and more directly, and thus with less scattering.
When we see a bright sun in a blue sky, the longer wavelengths of light are coming almost directly through the atmosphere to us, but the blue light has been bounced all over the place. It is coming at us from all directions, and so a bright sky seems uniformly blue.
Sunlight contains the whole spectrum of visible light. Since it is a mixture of all the colors, it appears to be white, or nearly so. But when the sun is low in the morning and evening, the light must travel a longer pathway through the atmosphere than it does at noon. Because light of longer wavelengths can do this more easily, the sun--and the sky near it--appear orange or red.
So there really is a certain analogy between a parakeet's feathers and a bright sky. In both cases, it's almost a trick of mirrors--what we are seeing is reflected light. For sky and bird, "blue" is what we perceive because light in the longer wavelengths passes more or less directly through or into the medium, while the shorter wavelengths are bounced around and returned to our eye as the color we know as blue.
