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Recently, in the United States, the moon and sun have become very deep red when they are near the horizon. The reason given for this is that there have been extensive wild fires in the Northwest. So, the smoke is apparently making the sun/moon red instead of yellow/orange. Why does smoke have this affect?

My guess is that the smoke is preferentially absorbing blue and green light emitted by the object, thus the balance of light is being tilted to the red part of the spectrum. If this is right, then why is the blue/green light absorbed more by the smoke? I mean the smoke should either block the light or not block it, right? Why should the color of the light make a difference?

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The answer is Rayleigh scattering.

Rayleigh scatter probability scales with the inverse 4th power of the wavelength - so shorter wavelengths have a much higher probability of scattering. This makes the sky blue, and the setting sun red.

As you add more (small) particles to the air, a relatively shorter path of light is sufficient to scatter the blue (shorter wavelength) components away - leaving you with a red sun.

You might be interested in this answer I wrote earlier - it gets into a lot more depth on the question of particles in the air, and how they affect the appearance of the sun.

A little diagram to help:

enter image description here

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  • $\begingroup$ And the above is independent of how the particular stuff making up the particles absorbs different colors of light... $\endgroup$ – DJohnM Sep 5 '17 at 21:57

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