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Raleigh Scattering - blue sky, red sunset

Raleigh Scattering - blue sky, red sunset - Physics Forum

Raleigh Scattering - blue sky, red sunset - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

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Old 05-10-2005, 01:53 AM
Jim Coe
Posts: n/a
Default Raleigh Scattering - blue sky, red sunset

I'm writing an eBook for computer graphics image workers and wanted to
mention atmospheric effects in a section on lighting simulation. To my
suprise, i couldn't find a clear explanation of sunrise/sunset sky and
sun color with Google!

Please correct/add to my current weak understanding:
Paraphrasing what i've found so far:

Blue sky:
The noonday clear sky is blue because of so called "Raleigh Scattering".
Raleigh Scattering describes how the shorter wavelengths of visible light
(blue end of the visible spectrum) are scattered much more than other
visible light, because they approach the size of air molecules. Since
these light frequencies are scattered in every direction, they illuminate
the sky with blue light.

Q1: Without Raleigh Scattering would the noonday clear sky be white?
Q2: What is it in the sky that is "illuminated" by the scattered light.
Is it not more correct to say that the air molecules re-radiate blue to
our eyes?

Red sunset/sunrise:
Best explanation i found was a poor one in a High School text (included
photo of a pretty sunset. Caption: "See how science is better than
Paraphrasing what i've found so far:
Again, it's Raleigh Scattering. At sunrise and sunset, the path of
sunlight to the observer is much longer. (so far so good). Then they say
the light is redder at sunset (no explanation) and then that sunlight is
again scattered by air molecules, as well as other particles, such as air
pollutants. (Very good. Sunsets are red because the light is red at

Q1: What is it about the longer pathlength that makes sunlight redder?

Q2: If the shorter noonday path scatters mostly blue (shorter
wavelength), i can imagine that a long enough column of atmosphere might
also scatter longer wavelengths, but what happened to the blue?. Why no
blue in sunset? I would expect scattered blue and scattered red to color
the sky violet, or if all wavelengths are now scattered, to make white

Q3: Is refraction now involved? How?

Q4: Since at sunset/sunrise the sunlight path nears the ground, i can
imagine that dust particles, atmospheric haze, smog and other
particulates play their part. If so how do they account for more red? And
what happened to the blue? Why isn't Raleigh Scattering still producing

Thanks all!

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Old 05-10-2005, 03:12 AM
N:dlzc D:aol T:com \(dlzc\)
Posts: n/a
Default Raleigh Scattering - blue sky, red sunset

Dear Jim Coe:

"Jim Coe" <[Only registered users see links. ]> wrote in message
news:Xns9651BFD724C38jimcoemindspringcom@207.217.1 25.201...

[Only registered users see links. ]
"Why is the sky blue?"
URL:[Only registered users see links. ]
URL:[Only registered users see links. ]

Rayleigh is the correct spelling. You'll get more hits that way.

No. Without Rayleigh scattering, the noonday sky would be pitch
black, just like it is on the Moon.

Your eyes.

Not really.

Most of the blue has been scattered, which leaves red.

The noonday sky is white, near the Sun. Near the horizon, the
sky is blue.

It is largely scattered to destinations closer to its first
encounter with the atmosphere.

As it is near the Sun at noon.

No. Not in scattering.

Blue is more energetic. The n value (index of refraction) for
blue in most materials is larger than for red... most materials
are active with blue, with less active with red.

Scattered higher in the atmosphere.

Rayleigh scattering doesn't "produce" blue, it "sorts out" (some)
blue. Leaving the balance of the spectrum behind.

David A. Smith

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Old 05-10-2005, 04:29 AM
Jim Coe
Posts: n/a
Default Raleigh Scattering - blue sky, red sunset

"N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote in

Thanks David - got it now. Very clear explanations.

Funny - the misspelling is so common that i got so many hits i thought it
the correct spelling. Might have something to do with the low quality of
what i found )

<jc snip>

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Old 05-10-2005, 08:10 PM
Prai Jei
Posts: n/a
Default Raleigh Scattering - blue sky, red sunset

N:dlzc D:aol T:com (dlzc) (or somebody else of the same name) wrote thusly
in message <UDVfe.4436$eU.3690@fed1read07>:

The light is reflected rather than re-radiated. The reflected light retains
its original colour (blue), re-radiated light would potentially have a
totally different spectrum.

The same mechanism accounts for blue eyes appearing blue. The iris is
actually unpigmented but is rather cloudy, with an intensely dark layer
behind it. The setup is exactly the same as with the sky, and the blue
arises by the same mechanism. But those same blue eyes appear *red* for the
same reason as the setting sun appears red, when viewed by transmitted
light, the familiar example being a flash photograph. The blue component of
the flashlight is scattered away and the red is reflected back to the

(I write from experience. "Prai Jei" means "blue eyes".)
A couple of questions. How do I stop the wires short-circuiting, and what's
this nylon washer for?

Interchange the alphabetic letter groups to reply
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Old 05-11-2005, 08:34 PM
Jim Coe
Posts: n/a
Default Raleigh Scattering - blue sky, red sunset

Prai Jei <[Only registered users see links. ].uk> wrote in
news:d5r439$m7q$[Only registered users see links. ].pol.co.uk:

Very, very interesting, Blue Eyes.
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blue , raleigh , red , scattering , sky , sunset

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