Why isn’t the sky violet, Daddy?

Kids often ask why the sky is blue. While physics textbooks correctly explain that short wavelengths of sunlight, such as the ones that we see as blue, more readily ricochet off atmospheric gas molecules into our eyes than do other solar wavelengths, that’s only a partial answer, says electrical engineer Glenn S. Smith of the Georgia Institute of Technology in Atlanta.

In fact, the measured spectrum of sunlight that makes it to our eyes after scattering off nitrogen and oxygen molecules remains roughly as intense at violet wavelengths as it does at blue ones, he notes.

So why do we see only blue? Chalk it up to certain combinations of colors affecting our retinal color-sensing cells, or cones in much the same way as pure colors do, Smith explains in the July American Journal of Physics. Using an example from the redder end of the spectrum, he notes that a combination of red and green lights of wavelengths 640 nanometers and 540 nm, respectively, appears to our eyes as a single yellow glow of 580 nm wavelength.

Smith then extrapolates to the broad band of wavelengths impinging simultaneously on cones from atmosphere-scattered sunlight. He calculates that our cones register that cacophony of predominantly violet-to-blue-green illumination as if it were pure, vivid 480-nm light toned down by weak, white light. In other words, it’s blue.

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