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
Ready? here comes the punch line. 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 color 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 the sky.
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.
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