WHY IS THE SKY BLUE, AND NOT VIOLET? The hues that we see in the sky are not only determined by the laws of physics, but are also colored by the human visual system, shows a new paper in the American Journal of Physics. On a clear day when the sun is well above the horizon, the analysis demonstrates, we perceive the complex spectrum of colors in the sky as a mixture of white light and pure blue. When sunlight enters the earth’s atmosphere, it scatters (ricochets) mainly from oxygen and nitrogen molecules that make up most of our air. What scatters the most is the light with the shortest wavelengths, towards the blue end of the spectrum, so more of that light will reach our eyes than other colors. But according to the 19th-century physics equations introduced by Lord Rayleigh, as well as actual measurements, our eyes get hit with peak amounts of energy in violet as well as blue. So what is happening? Combining physics with quantitative data on the responsiveness of the human visual system, Glenn Smith of Georgia Tech (glenn.smith@ece.gatech.edu) points to the way in which our eye’s three different types of cones detect color. As Smith shows, the sky’s complex multichromatic rainbow of colors tickles our eye’s cones in the same way as does a specific mixture of pure blue and white light. This is similar to how the human visual system will perceive the right mixture of pure red and pure green as being equivalent to pure yellow. The cones that allow us to see color cannot identify the actual wavelengths that hit them, but if they are stimulated by the right combination of wavelengths, then it will appear the same to our eyes as a single pure color, or a mixture of a pure color and white light. (Smith, American Journal of Physics, July 2005)

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