The sky is blue — physicists tell us — because blue light in the sun’s rays bends more than red light. But this extra bending, or scattering, applies just as much to violet light, so it is reasonable to ask why the sky isn’t purple.

The answer, explained fully for the first time in a new scientific paper, is in the eye of the beholder.

“The traditional way that people teach this subject is that sunlight is scattered — more so for shorter wavelengths than for longer ones,” says Glenn Smith, an engineering professor at Georgia Tech. “The other half of the explanation is usually left out: how your eye perceives this spectrum.”
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While writing a physics textbook some years ago, Smith noticed that physiology usually gets short shrift, even though the spectrum of skylight — when analyzed — is about equal parts violet and blue.

Smith has written an article for the July issue of the American Journal of Physics that puts the physics of light together with the physiology of human vision.

“This is nothing that people who work with eyes haven’t known for a long time,” Smith told LiveScience. “I just had not seen it all in one place before.”

By Michael Schirber

More here.