MadSci Network: Physics

Re: Are there other atmospheres that might scatter other colors of light?

Area: Physics
Posted By: John Christie, Faculty, School of Physical Chemistry, La Trobe University, Bundoora, Victoria, Australia
Date: Thu Dec 19 19:20:28 1996
Message ID: 849985543.Ph

The simple answer is no. "Rayleigh Scattering", where the scattering centres
are significantly smaller than the wavelength of the light involved, obeys
a law where the intensity of the scattered beam is proportional to the 4th
power of the frequency of the light. Thus the scattered beam of blue light
wavelength 460 nm is 4 times as strong as the scattered beam of red light
wavelength 650 nm {650/460 = approx sqrt(2); sqrt(2)^4 = 2^2 = 4.} 

Actually, violet light is scattered more than blue light, and ultraviolet
light more still, which is why you can get very severe sunburn from
skylight on bright hazy days, even if you stay in the shade from the direct
sun. But when you average over all of the visible spectrum, the wavelength/
frequency discrimination means that yellowish-white sunlight is scattered as
blue skylight.

With larger particles, different scattering laws apply, and there is 
generally much less wavelength discrimination. Clouds appear white (if they
scatter a lot of light toward the ground), or grey (if they do not). Water
droplets or ice crystals in clouds or fogs are too large to be Rayleigh
scatterers. You can also get special effects at particular scattering angles,
especially when larger scatterers are very uniform in size and/or shape.
Rainbows, and "glory scattering", where you see bright rings or disks
surrounded by a greyer area, are examples of this.

But if, as I suspect, you are trying to envision a planet where the sky would
be green, or pink, or yellow, this could only happen as a result of
coloured gases, or large particles in the atmosphere; it is not a possible
outcome of the Rayleigh scattering mechanism that colours our Earthly sky.


An afterthought: In 1992 & 1993 we had skies here that were quite pink,
when you looked fairly close to the sun. They were the result of a high level 
aerosol of sulfuric acid that was present after a couple of major volcanic
eruptions. But the sky still looked deep blue if you looked a good distance 
away from the sun.

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