Why is the sky blue? Not as simple as you might think ...

**rinselberg** · 03-24-2007, 01:41 PM

Why not violet? If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works. We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths. As they are stimulated in different proportions, our visual system constructs the colours we see.

Response curves for the three types of receptor cones in the human eye.

When we look up at the sky, the red cones respond to the small amount of scattered red light, but also less strongly to orange and yellow wavelengths. The green cones respond to yellow and the more strongly-scattered green and green-blue wavelengths. The blue cones are stimulated by colours near blue wavelengths which are very strongly scattered. If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge. However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colours appear blue with an added red tinge. The net effect is that the red and green cones are stimulated about equally by the light from the sky, while the blue is stimulated more strongly. This combination accounts for the pale sky blue colour.

It seems unlikely that it is just coincidence that our vision is parameterized to perceive the sky as a pure (blue) colour. Our evolution has been shaped by our environment; and the ability to separate natural colours [from background or sky] ... is probably a survival advantage.

Why is the sky blue? Philip Gibbs; May 1997.

The question was revisited in 2005; to wit:

The usual answer to the question "Why is the sky blue?" is based only on Rayleigh scattering from the molecules in the atmosphere and makes little mention of the contribution of color vision. We supplement this answer with a quantitative discussion of the role color vision plays in determining the appearance of the daytime sky. The anatomy of the human eye is reviewed, and its response as a function of wavelength is described via the spectral sensitivities of the cones. Color matching is examined for a mixture of monochromatic lights and for the spectrum of the daytime sky. The spectral irradiance of skylight is shown to be a metameric match to unsaturated blue light.

Human color vision and the unsaturated blue color of the daytime sky. Glenn S. Smith; School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332. American Journal of Physics; July 2005. Complete article with diagrams reproduced online in PDF format.

Smith's article was reported in the popular press:
Blue Skies Only In the Eye of the Beholder. Michael Schirber; LiveScience; July 2005.

But there's another twist.

Everyone’s eyes are wired differently
Images of retinas show wide diversity in distribution of light-sensing cones

These color-coded images of living human retinas show the wide diversity in the number of cones sensitive to different colors. The image on the left shows the retina of a person who has few red-sensing cones, and the one on the right is of someone with far more red cones.

On average, there are 7 million cones in the human retina, 64 percent of which are red, 32 percent green, and 2 percent blue, with each being sensitive to a slightly different region of the color spectrum. At least that's what scientists have been saying for years.

A 2005 report from the Center for Visual Science at the University of Rochester found that people recognized colors in the same way - [but] there is enormous variability, sometimes up to 40 times, in the relative number of green and red [photo receptor]cones in the retina from one person to another ... [even among people who are considered to have fully functional color discrimination.]

[This] suggests that there is a compensatory mechanism in our brain that negates individual differences in the relative numbers of red and green photo receptor cones that are observed.