why is the sky blue and not violet?

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)

courtesy aip

7 thoughts on “why is the sky blue and not violet?

  1. this article is quiet difficult to understand.can u make it more simplier. i mean the “visual system of human”-this part,can it be more elaborated. thanks

  2. i just want to know why the sky is blue not violet.violets wavelength is smaller than blue.so it should be much more scattered than blue.so the sky should be looked violet.not blue.pls help me to understand it..

  3. I think author has failed to answer the real question. Sir/ma’am please answer the question why does sky appear blue when it should appear violet on the basis of Rayliegh’s scattering

  4. I beg your pardon everyone, but the post does in fact answer the question. It clearly states that the sky appears blue because of the way the scattered light triggers cones in our eyes. Due to the scattering, we get exposed to the high energy end of the spectrum the most, but it’s not just blue and violet that enter our eyes. The unique mix of all the other colors that aren’t scattered as much but are still present are interpreted by our brains as pure blue, much in the way that pure red and pure green get interpreted as pure yellow.

  5. I think the post does actually answer the question, it’s just that the answer requires an understanding of optics and biology.

    The answer, if I am reading it correctly, is that on top of the scattering effects on the light wavelengths, you also have to account for the relative sensitivity of the retinal cone pigments to different wavelengths of light and overlaying this is the “perception” of colour. The example used is that we don’t actually detect “yellow” per se, but the response of our brain to the stimulus on our photoreceptors allows us to “perceive” yellow (or so the rough idea goes). Therefore, in the same way, a mixture of violet and blue light, given, scattering, relative intensities from the sun and absorbtion in the atmosphere leads to the perception that the mixture of shorter wavelength light our eyes detects coming from the sky on a bright clear day is “percieved” as being blue.
    This is, admittedly a much “messier” picture than physicists are usually happy with, but there is a lot of stuff on the internet from the likes of astronomers etc. who do beautiful work on response to intensity and colour of light which is relevant to the topic above.

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