Posted May 24, 2005 by Danny B. [Musicians' Guild (U150368); Science EXplained (A4108330); Curator; Scavenger]
OK, let's get the  rolling...
Why *is* the sky blue? I vaguely understand this (I think), but it would be nice to see it simply explained once and for all 
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Posted May 24, 2005 by icecoldalex
Well to start you off:
White light from the sun contains all of the colours of the rainbow.
The particles in the atmosphere scatter the light (like what happens when you pass white light through a glass prism to get a rainbow). The blue light is scattered more than the red light and is directed towards the Earth.
So when we look up we see mainly blue.
Anyone else want to contribute?
Alex.
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Posted May 24, 2005 by Not him. Not here. Slightly pished
does this mean that red light is still scattered, but less?
does this mean that the area closer to the dirct line between us and the sun is redder? (or is closer to the red end of the spectrum)
does it mean that the sun appears whiter when seen from space?
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Posted May 24, 2005 by icecoldalex
<<does this mean that red light is still scattered, but less?>
Yes
<<does this mean that the area closer to the dirct line between us and the sun is redder? (or is closer to the red end of the spectrum)>>
I'm not sure what you mean by this but it does depend where you are on the Earth in relation to where the sun is. At sunset you see the sky redder. The blue part is going to the part of the Earth that is at midday time or daytime anyway. Sorry this is a bit garbled I'll try and find a diagram for you.
<<does it mean that the sun appears whiter when seen from space?>>
Not sure about this one but I think so, yes.
Alex
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Posted May 24, 2005 by Arnie Appleaide - Inspector General of the Defenders of Freedom
For visible light scattering from molecules, the following equation can be derived (and is found to be experimentally verified):
I = A/(lambda^4)
I is the intensity of light scattered
A is a contanst
lambda is the wavelength of light
lambda^4 is lambda raised to the fourth power.
This equation means that the shorter the wavelength, the more likely the light is to be scattered. Blue is at the edge of what we can see, so that is the light we see scattered.
The other thing to consider is that when you look at the sky and see blue, you're seeing blue light from the sun, which struck a molecule in the atmosphere, and got scattered such that it then struck your eye. So all the light from the sun is passing by those molecules - its just that the higher wavelengths (green, red) are more likely to not scatter and keep going.
to answer the other questions:
"does this mean that red light is still scattered, but less"
yes - we could probably detect that red light with an advanced scientific device, but not with our eyes
"does this mean that the area closer to the direct line between us and the sun is redder"
No - it means that it is more closely to match the composition of wavelengths that are coming from the Sun itself - the most common being "yellow"
"does it mean that the sun appears whiter when seen from space"
No - the sun's color is not affected by this type of scattering. The intensity of light traveling directly from the sun to your eye is much, much greater than that of scattered light from sky to your eye.
I read up on this from the book "Classical Electrodynamics" by Schwinger, Deraad, Milton and Tsai
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Posted May 24, 2005 by Gnomon
<<does this mean that red light is still scattered, but less?>
Yes. That's why the sky often turns red at sunset. It's only when the sun is shining through lots of atmosphere (as it does when on the horizon) that the scattering of the red is enough to overpower the blue.
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