In BA the chapter on why is the sky is blue the simple explanation is random scatterings of blue light. The accompanying diagram shows various rays of light taking random paths to the observers’ eyes.

But can this be so? If the light travelled in random directions before reaching the eye then it must surely pass in front of distance objects to the observer. For instance a white cloud some 15-20 kms away would have blue light scattered in front of it. Is this argument sensible?

But the blue light from the sky always appears to come from behind any distance object! How can this behaviour and the random light scattering theory both be reconciled?

Walter

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Walter Zambotti

Valid question. The issue is the degree of scattering that occurs. Over short distances, there is not enough scatter to be significant. It's only when you have the entire thickness of the atmosphere working to scatter light do you get the blue sky.

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Freedom For Fission A breath of fresh Iodine-131

If the degree of scatter is distance based then what explains the difference of the level of blue-ness between the sky when looking directly up and the sky when looking at the horizon.

The sky when viewed at the horizon would be thicker than the sky directly overhead. So more distance and subsequently more degree of blue! But this is not the case.

In the interests of expedience I offer a possible counter argument.

So the light at the horizon would have to be refracted at 90-degree angles to the observer while the light from overhead would require no refraction.

It may be that the intensity of blue light is also based on the angle of the light entering the sky.

Hmm!!!

Only true if the SUN is directly overhead. Even when the sun is on the horizon the sky looks bluer overhead than at the horizon. In fact the opposite horizon looks bluer than the horizon with the sun. This would imply a 180-degree refraction of the blue light.

Ok so lets progress the argument!

NEXT!!!

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Walter Zambotti

Clouds aren't that far away. Mountains are. Distant mountains appear blue.

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'Sir........, I don't like these numbers.'
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As others have noted, a lot of the scattering that produces the blueness of the sky occurs high in the atmosphere, above most of the clouds. What's more, only a small fraction of the incoming blue light is scattered -- most comes through just like the rest of the spectrum.

When you compare the overhead view to the horizon view, other factors come into play. Besides the scattering by N2 that produces blueness, you also get significant absorption due to the much greater thickness of the atmosphere along the horizon sightline. This absorption varies a lot depending on the amount of dust, water vapor, and other variable components of the atmosphere. That's why the sun turns red at sunset -- lots of dust (etc.) absorbing the blue end of the spectrum.

So it's a combination of effects that cause the blueness of the overhead sky to look more washed out at the horizon.

mik sawicki criticizes that diagram in his comment number 8. As he says, the blue photons passing through the entire atmosphere are only scattered about once on average (or else we wouldn't see polarized light at ninety degrees from the Sun, right?)

I am confused. I think my definition/education on polarization may be incorrect. I will check the latest accepted, and return.

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'Sir........, I don't like these numbers.'
'Then hire somebody that can change them!'
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Clouds appear white because when you look at a cloud you see all colours of light being equally reflected and scattered by the cloud in addition to the blue light being scattered towards you by the intervening atmosphere. The sky, however, doesn't reflect light (not much, anyway): it only scatters it, and most of the scattered light is blue. So when you look at the sky, all you are seeing is the scattered blue light. It does not really matter how far up the cloud is. The Moon is even farther away and it doesn't look blue when you see it during the day! It looks white because it reflects all wavelengths of light equally.

Distant mountains do indeed look bluish because of airlight (ie blue light scattered by the intervening atmosphere). But the mountains were dark to begin with. That is, they were not reflecting much light in the first place; so the airlight is not drowned out by other light. Snowcapped mountains do reflect a lot of the light falling on them, and they look white, even when they're far away.

I stand corrected.

And welcome to the board Eroica. =D>

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'Sir........, I don't like these numbers.'
'Then hire somebody that can change them!'
("`-/")_.-'"``-.,,
\. . `; -._( );, `)
(v_,)' _ )`-. \ ``'`
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edit.
Wrong forum ops:
But I will put something here later on.

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Thank god for cancer.

Thanks for the warm welcome, Pinemarten. Sorry if my first post sounded a bit preachy!

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Preachy?
I am here to learn like many others.
I should link you to some of my 'preachy' posts. :wink:

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'Sir........, I don't like these numbers.'
'Then hire somebody that can change them!'
("`-/")_.-'"``-.,,
\. . `; -._( );, `)
(v_,)' _ )`-. \ ``'`
_.- _..-/ /((.'
((,.-' ((,/

One other thing. It's disappointing to read a chapter on why the sky is blue without once coming across John Tyndall's name. On page 41, Phil credits Lord Rayleigh with discovering that air molecules scatter blue light preferentially "in the mid-1800s." But Rayleigh was born in 1842, so he was only a kid in the mid-1800s. In fact, it was the Irish physicist John Tyndall who first proved experimentally that blue light is scattered more than other colours. That was in 1869, two years before Rayleigh's first publication on the same subject.

In his New Guide to Science, the late great Isaac Asimov correctly credits the Irishman with the discovery and calls the phenomenon The Tyndall Effect.

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I once saw a painting of a landscape. It was a very beautiful, clear painting which looked completely normal, except that the sky was green! Talk about surreal.

It looked strange, but was also quite tranquil and peaceful.

The sky on Mars is pink isn't it?

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Yahbut, Tyndall was studying suspended particles, which do not account for the blue of the sky.

Rayleigh generalized the theory ("Rayleigh scattering"), and Einstein used that to show that it was air molecules themselves, not suspended particles, that produce the blue of the sky. So, in a sense, Phil is right.

Thanks for that, kilopi, and apologies to Phil. I stand corrected. The history of it is explained quite well here:

http://math.ucr.edu/home/baez/physic.../blue_sky.html

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That's what I always thought, but at the Tyndall link I just gave it says that this is only after a dust storm:

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This makes the most sense. Mie Scattering has not been mentioned yet but it is noteworthy as it works with the larger particles. Molecules around .03 micrometers cause the Rayliegh/Tyndall scattering especially in the shorter wavelengths (by the 4th power). So, blue is the winner here. Also, Rayliegh scattering is somewhat omni directional so that it "goes all over the place". This is why blue is so predominant overhead when the sun is not. Mie Scattering is not so omni directional and it does not favor much of any one color. So you will see more white light around the area of the Sun. Both of these phenomena are "inelastic" in that the light is "bent" and not absorbed and re-emitted. Raman scattering and others are not inelastic as I understand but also contribute more to a whitish look (I think).

The thickness of the atmosphere is not that crictal compared to the dust factor which creates more scattering favoring white light. The sky appears bluer on top the mountains or in a jet since there is less dust scattering that washes out the pretty blue.

A good example of this is cigarette smoke. What color is it? It is the Rayliegh scattering that makes it blue as the particles are small enough to cause the effect. What color is the smoke after you exhale it (hopefully not you or me)? It is white because the particles are now larger thanks to the persons body moisturizing.

What I'd like to know is...what really is the color of the sun (without our atmosphere and at a distance where the color cones aren't overloaded?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

There is a web site called "why is the sky blue". why blue

It has 5 quizes of 10 related questions each. Unfortunately, some of their answers are wrong. :roll: Can you name them?

One statement I am unsure of....
At least, I don't think this is true.

Here is a poem from that site. What's wrong with this one?

Why the Sky is Blue, a Poem by John Ciardi

I don't suppose you happen to know
Why the sky is blue? It's because the snow
Takes out the white. That leaves it clean
For the trees and grass to take out the green.
Then pears and bananas start to mellow,
And bit by bit they take out the yellow.
The sunsets, of course, take out the red
And pour it into the ocean bed
Or behind the mountains in the west.
You take all that out and the rest
Couldn't be anything else but blue.
Look for yourself. You can see it's true.

Pretty, but, pretty bad, too. :-?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

The red (orange) color of the sun at sunset it is usually considered a consequence of yellow and green light scattering, which becomes also important due to the longer way of light in our atmosphere at sunset; indeed, the evening sky looks blue above and becomes yellow to the horizon.

My question is: should not be a green portion of sky between the blue and yellow ones? The green light it is scattered less than blue, but more than yellow; also our eyes are very sensitive to green.