Re: why is the sky blue?
Area: Earth Sciences
Posted By: Max Sang, Collective Enigma Elucidator
Date: Sun Nov 24 03:27:54 1996
Take a big glass of cold water and put it on a table with a bright light
shining through it. A black table or a piece of black material is best.
Take a tiny amount of milk - just a few drops, and skimmed
milk is best although other types should work OK- and drop it carefully
into the glass. Does the water look white? Or does it look a little blue?
Next look through the water at the light. Does it look blue? Or a bit red/brown?
This should give you a clue about the sky. White light from the sun passes
through air fairly well, but there is a lot of air between here and the outer
reaches of the atmosphere, and some of the light gets scattered. Scattering
means that the light waves get their direction changed by the molecules and
dust particles in the air. The thing is, all waves have something called
wavelength, which is a measure of the distance between the crests
of the waves. The wavelength of red light is the longest, and blue light
has the shortest wavelength, with orange, yellow, green, blue and indigo
in between. The key to this answer is that short wavelengths scatter better
from small things than long wavelengths do. If we have lots of tiny things
floating about in the water or air, like the tiny dots of fat and protein in
the milk or tiny dust particles in the air, the blue-ish light gets scattered
quite a lot, but the red-ish light gets scattered less. Take a breather, I know
this is a long answer....
Ready? here comes the punchline. Lots of white light sets out from the sun and
passes through the atmosphere. The deeper it gets, the more of the blue light
gets scattered away from its original path. By the time it reaches the ground
the white light has gone a bit yellowish, the colour of what we call 'sunlight'.
The blue light just bounces about, getting scattered like crazy, and all we
see, instead of the starry diamond skies we see a big blue haze, which we call
There are a couple of other things you might like to know about this. When the
sun gets really low in the sky, the light has to go through much more
air, which is why the sun looks bright red (never look at the sun directly
unless it's very low, near to sunset - you could go blind). Ever wondered why
clouds are white? The droplets of water of which clouds are made are much bigger
than the wavelength of red light, so it gets scattered just as much as the blue
light does, so all of the white light scatters, not just the blue part.
Hope this helped you to understand it!
Max Sang (CERN, Geneva, Switzerland)
P.S. I'm from Britain, which is why I write colour instead of color!
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