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Name: macmillan
Status: N/a
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

Why is the sky blue? Why is glacial ice blue? What common properties cause them both to appear blue?

In this case the reflected color is a measure of the size of the centers that are scattering the light. The fact that a sunset is red and the sky is blue indicates that the scattering centers for light are not big enough to scatter red light, but are big enough to scatter blue light or shorter wavelength light. Usually dust is regarded as a contributor, but molecular sizes are also involved. I believe.

Sam Bowen

Air molecules scatter (reflect) the shorter wavelengths (violet, blue, and green) more effectively than the other colors. As you look at the sky the violet, blue, and green strike your eyes from all directions, combining to make blue. Pure ice crystals also scatter light so as to give a blue color, the purer the ice the bluer the color. Glacier ice is purer than other ices. A sunset is red because the light is going through so much air that the short wavelengths are scattered away and only the longer wavelengths are bent into your eyes. Dust particles scatter all wavelengths about the same so a really dusty sky looks white.

Mark Fernau

The reasons for why the sky is blue and why glacial ice is blue are different. The sky is blue because the air molecules scatter the short (violet/blue) wavelength part of the visible spectrum more strongly that the long wavelength (red) part. For an excellent discussion, see "Colors of the Sky," C. F. Bohren and A. B. Fraser, The Physics Teacher, May 1985, pp. 267-272.

Glacial ice is blue because of the selective absorption by the ice of the red part of the visible light spectrum. For a nice discussion, see the following web site:

Dr. Lawrence D. Woolf
General Atomics
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