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Name: Daniel
Status: student
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Question:
Why isn't the sky purple, since purple has an even shorter wavelength than blue and ought to be more easily dispersed by the air molecules?


Replies:
Because there is not much purple light in sunlight. Although violet is scattered more efficiently than blue, there are many more incoming blue photons to scatter.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Rayleigh-scattering's preference for short wavelengths is gradual. There is a power law formula; I forget what the exponent-number is. But, red and blue are not terribly different from each other in wavelength. The wavelength ratio is perhaps 0.45 / 0.63 = ~0.68. Green is even closer to blue, 0.45 / 0.53 = ~0.84. Raising those ratios to the power appropriate for scattering still leaves a broad mix of colors with more blue but some red and green, not solely the pure blue-violet you are thinking of. Think of sky-blue as blue+white, or a level white spectrum mildly tilted towards blue. This is especially true when the sky has some Mie scattering due to particles larger than molecules and then the sky takes on a whitish rather than rich blue color. A portion of the pre-dawn sky high over the eastern horizon sometimes has a deep clear blue, which represents about the shortest average wavelength you will get to see in the sky.

I would like to suggest to you a distinction between the colors "purple" and "violet". "Purple" is the color one experiences from a true mix, blue light plus some actual red light (620-700nm). Pure short-wavelength (~400nm) blue light definitely causes the color "violet" but cannot quite reach a ripe "purple". ("Indigo", something like a navy-blue glow, is more commonly experienced, because the blue light one usually gets has a spread from 400-440nm.) Our 3 visual color-responses have some cross-sensitivity to other wavelength bands, and I think that far blue light can stimulate our visual red response a little more than it stimulates our visual green response, and so far-blue light ends up seeming violet, i.e., half-way between deep_blue and purple. It is a good understanding to have, but given that the eye has cross-responses and color-names are difficult to standardize, I would not be dogmatic about it.

Jim Swenson


Daniel,

First, particles, as well as molecules, scatter light in the atmosphere, although generally to a lesser extent, unless you are located in a highly polluted area such as an urban area.

Blue and violet are scattered preferentially, but violet hues make up a much smaller portion of the solar spectrum than do blue hues. Therefore, scattered blue hues dominate over violet and the sky appears blue. For a very clean atmosphere (rural area), the sky can appear dark blue, whereas in an urban area, where particle scattering is very important, the sky appears light blue.

David R. Cook Ph.D.
Argonne National Laboratory



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