PDA

View Full Version : Questions on why is the sky blue?



wzambotti
2003-Apr-30, 03:07 AM
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 observersí 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

Glom
2003-Apr-30, 02:11 PM
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.

wzambotti
2003-May-01, 02:11 AM
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!!!

Pinemarten
2003-May-01, 02:22 AM
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


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

Donnie B.
2003-May-01, 02:28 PM
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.

kilopi
2003-May-01, 03:35 PM
mik sawicki criticizes that diagram (http://www.badastronomy.com/phpBB/viewtopic.php?topic=3774&forum=9&3) 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?)

Pinemarten
2003-May-03, 12:50 PM
I am confused. I think my definition/education on polarization may be incorrect. I will check the latest accepted, and return.

Eroica
2003-Sep-22, 04:28 PM
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.

Pinemarten
2003-Sep-23, 07:31 AM
I stand corrected.

And welcome to the board Eroica. =D>

browndwarf
2003-Sep-23, 08:07 AM
edit.
Wrong forum :oops:
But I will put something here later on.

Eroica
2003-Sep-24, 07:15 AM
Thanks for the warm welcome, Pinemarten. Sorry if my first post sounded a bit preachy!

Pinemarten
2003-Sep-24, 08:11 AM
Preachy?
I am here to learn like many others.
I should link you to some of my 'preachy' posts. :wink:

Eroica
2003-Sep-25, 03:11 PM
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.

Richard of Chelmsford
2003-Oct-04, 01:48 PM
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?

kilopi
2003-Oct-04, 02:33 PM
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.
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.

Eroica
2003-Oct-04, 02:41 PM
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/physics/General/BlueSky/blue_sky.html

Eroica
2003-Oct-04, 02:46 PM
The sky on Mars is pink isn't it?

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


Images sent back from the Viking Mars landers in 1977 and from Pathfinder in 1997 showed a red sky seen from the Martian surface. This was due to red iron-rich dusts thrown up in the dust storms occurring from time to time on Mars. The colour of the Mars sky will change according to weather conditions. It should be blue when there have been no recent storms, but it will be darker than the earth's daytime sky because of Mars' thinner atmosphere.

George
2003-Oct-06, 02:52 PM
So it's a combination of effects that cause the blueness of the overhead sky to look more washed out at the horizon.

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?

George
2004-Apr-03, 05:22 PM
There is a web site called "why is the sky blue". why blue (http://www.why-is-the-sky-blue.org/sky-blue-quizzes.html)

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....

High concentrations of plankton make the ocean appear blue-green.
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. :-?

Baloo
2004-May-16, 12:47 PM
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.

George
2004-May-17, 09:40 PM
Welcome aboard the board, Baloo. =D>

A very good question, too.


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.

Yes. You are correct and this scattering is called Rayleigh Scattering. There are several factors which affect light in regards to Rayleigh scattering and distance through the atmosphere is one of them. Light scatters as the 4th power of it's frequency, so blue light scatters about 10x more than red.


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.
This logic just doesn't want to work with our eyes, unfortunately I suppose. Stars behave close to blackbody emitters. This means that even if their peak emission is green light, they will also emit a lot of blue, yellow and even red. Also, our "red" and "green" sensitive cones are much more sensitive than our "blue" cones. Another factor is our atmosphere's ability to abosorb and scatter blue and green light more than the other colors. The sun losses about 1/2 of it's blue light as it goes through our atmosphere. The result is a fairly flat spectrum when seen here on the ground. These three factors (other colors in the light, our unbalanced sensitivity and the flatening of the higher fequency light [blue & green]), do not allow "green to be seen".

I still have hope that, above our atmosphere, a good bit more color might exist in stellar observation including our Sun. Since it peaks in the blue-green will it look any different? It is not any easy question, surprisingly. [I am somewhat alone on this issue and I am not an expert by any stretch, but, I still am curious about this.]

Baloo
2004-May-17, 11:38 PM
Welcome aboard the board, Baloo

Thanks :wink:


This logic just doesn't want to work with our eyes, unfortunately I suppose. Stars behave close to blackbody emitters. This means that even if their peak emission is green light, they will also emit a lot of blue, yellow and even red. Also, our "red" and "green" sensitive cones are much more sensitive than our "blue" cones. Another factor is our atmosphere's ability to abosorb and scatter blue and green light more than the other colors. The sun losses about 1/2 of it's blue light as it goes through our atmosphere. The result is a fairly flat spectrum when seen here on the ground. These three factors (other colors in the light, our unbalanced sensitivity and the flatening of the higher fequency light [blue & green]), do not allow "green to be seen".

I still can't find any reason for the green light to behave different than blue and yellow light. And I don't think that Earth's atmosphere is treating green light preferentially (like high absorption or high scattering- at list I did not found anything about that on a quick search on the internet ). But this one could be easily solved if we could have a look at a solar spectrum acquired with an Earth based spectrometer and one acquired with a space based one. (again, quick search, but nothing). Maybe someone has a link...
Maybe there is something with our vision, but I don't know any other example of such a green "blindness". :-k

George
2004-May-18, 02:28 PM
I still can't find any reason for the green light to behave different than blue and yellow light. And I don't think that Earth's atmosphere is treating green light preferentially (like high absorption or high scattering- at list I did not found anything about that on a quick search on the internet ). But this one could be easily solved if we could have a look at a solar spectrum acquired with an Earth based spectrometer and one acquired with a space based one. (again, quick search, but nothing). Maybe someone has a link...
Maybe there is something with our vision, but I don't know any other example of such a green "blindness". :-k

These might help...
This one gives a lot of history at first but becomes very helpful. I like his quote...."It's not easy being green" - Kermit the Frog. here (http://www.strangehorizons.com/2004/20040510/green.shtml)

This shows how much more higher frequency light is diminished traveling through our atmosphere.
here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

You will find many sites for Rayleigh Scattering showing how light is scattered by the fourth power of it's frequency.

This site is quit interesting as it shows only variations of red and blue to be observable by blackbody radiators. It takes into consideration the performance characteristics of our eyes and concludes our sun is ......Peacy Pink! Peach Pink Sun (http://casa.colorado.edu/~ajsh/colour/Tspectrum.html)
You would think this result alone would cause great stir in the world. If you are stirred by this, let me know 'cause that will make at least two of us stirred. :)



Personally, I would like to simulate the irradiation curve above our atmosphere and "see" what color is observable.

George
2004-May-18, 02:33 PM
The solution would be if you know any cosmonauts which would be willing to take up one of my 19 cent strobes. This would settle the issue. [The sun is too intense to get a true color rendering by anyone orbiting Earth.]

Baloo
2004-May-18, 07:35 PM
This shows how much more higher frequency light is diminished traveling through our atmosphere.
here

Well, it seems that the red light it is the most favorised. But, even if the green light it is more absorbed, is not so dramatically affected. Anyway, it is far from beeing totally absorbed. (this is obvious, there is plenty of green around us, but I'm still talking about the greenless evening sky #-o ).


This site is quit interesting as it shows only variations of red and blue to be observable by blackbody radiators. It takes into consideration the performance characteristics of our eyes and concludes our sun is ......Peacy Pink! Peach Pink Sun

Don't know if you noticed, there is another thread around here about "the yellow sun". Obviously, they should rename it. :)
It seems that the human eye has a higher saturation threshold for yellow, and perhaps that's why the sun (which has a strong enough emission at all visible wavelength to saturate our eyes) it looks yellow.
The blackbody trajectory from the RGB diagram is based on the measures made above our atmosphere. Perhaps our atmosphere is shifting the blackbody trajectory a little bit to yellow (or white), which is not so far (on the diagram) from pink.


The solution would be if you know any cosmonauts which would be willing to take up one of my 19 cent strobes

Unfortunately, don't know any...yet. :)

George
2004-May-18, 09:11 PM
The scattering and absorption does "bleach" out more than half of the violet and above frequencies, about half of the blue and green and almost half the yellow. The big losers being green to violet. The result is somewhat even color intesities and hence...a white sun (once far above the horizon). A strobe will reveal the sun to be white when not near the horizon.


Don't know if you noticed, there is another thread around here about "the yellow sun".

There is a S.A.D. story there. :)

The reason we see so much green around us (e.g. grass, trees) is due to their absorption of the other colors. It would be interesting if something green is because all other colors but yellow and blue were absorbed. I am not knowledgeable in color science, however, I try to be colorful. :wink:

Baloo
2004-May-18, 09:53 PM
It would be interesting if something green is because all other colors but yellow and blue were absorbed

Not quite. Something is green because is absorbing ALL the colors (including yellow and blue) but green. The combination blue+yellow=green (or the others combinations used in television between Red, Green, Blue) is a kind of trick for our eyes; a spectrometer will detect blue and yellow (and no green) even we see green. Mmm...could be a similar reason for which we see the sun yellow even if the solar spectrum is relatively flat between green and red? #-o

George
2004-May-18, 11:04 PM
It's a little tricky as they eye/brain is part of the equation. You might enjoy the "yellow sun" thread as to why it is not yellow (i.e. when overhead).

lyndonashmore
2004-Jul-26, 09:44 PM
I feel that we have lost the original post. Why is the sky blue?
The question we should ask is, why is it not violet?
we all know about Lord Rayleigh and that the higher the frequency the more the light is scattered. Violet is a component of the spectrum and it has a higher frequency than blue. Ergo, it will be scattered more. So the sky should be violet.
If you don't want to accept this, then one must accept that the sky should at least be 'banded' with red nearest the Sun and Violet, scattered the most, the furthest away.
Why is VIOLET always discriminated against. this is colour predudice!

George
2004-Jul-26, 10:48 PM
The blackbody trajectory from the RGB diagram is based on the measures made above our atmosphere. Perhaps our atmosphere is shifting the blackbody trajectory a little bit to yellow (or white), which is not so far (on the diagram) from pink.
It would be hard for me to swallow "pink", unless we are talking about lemonade. :)

The atmosphere flattens our Sun's blackbody curve so we get the same amount of light from blue to red. If all of our sets of three color cones all fully saturated, I believe we would see only white.



The solution would be if you know any cosmonauts which would be willing to take up one of my 19 cent strobes

Unfortunately, don't know any...yet. :)[/quote]
Shucks, you're the closest one to them. :-?

George
2004-Jul-26, 11:06 PM
I feel that we have lost the original post. Why is the sky blue?
The question we should ask is, why is it not violet?
we all know about Lord Rayleigh and that the higher the frequency the more the light is scattered. Violet is a component of the spectrum and it has a higher frequency than blue. Ergo, it will be scattered more. So the sky should be violet.
If you don't want to accept this, then one must accept that the sky should at least be 'banded' with red nearest the Sun and Violet, scattered the most, the furthest away.
Why is VIOLET always discriminated against. this is colour predudice!

Rayleigh scattering works as the 4th power of the frequency. Violet scatters about 50% more than blue but you have less than half as much violet light to begin with. Added to this problem is your "blue" color cone which may be stronger in blue than violet. (There seems to be some question on this last point).

Maybe there are some elements which would abosrb blue but not violet so we could release them and see a violet sky. 8)

lyndonashmore
2004-Jul-27, 11:04 AM
Rayleigh scattering works as the 4th power of the frequency. Violet scatters about 50% more than blue but you have less than half as much violet light to begin with. Added to this problem is your "blue" color cone which may be stronger in blue than violet. (There seems to be some question on this last point).

Maybe there are some elements which would abosrb blue but not violet so we could release them and see a violet sky. 8)
The trouble is that apart from trying to bring cosmology down to Earth by showing that the BB theory is a load of codsmology, I am also into photography and I have always done my own colour printing. Any good printer will tell you that if you want to see if your final print of a photo has a 'colour cast' then one looks at the sky on the picture. Sky blue is a very particular colour so particular that I, as a natural sceptic, find it hard to believe that it is by 'chance'. An energy level would be much more understandable.

tjm220
2004-Jul-27, 07:54 PM
The trouble is that apart from trying to bring cosmology down to Earth by showing that the BB theory is a load of codsmology...

What is the definition of "codsmology" and why is your comment not in ATM?

lyndonashmore
2004-Jul-27, 10:18 PM
The trouble is that apart from trying to bring cosmology down to Earth by showing that the BB theory is a load of codsmology...

What is the definition of "codsmology" and why is your comment not in ATM?
What is ATM? to me it is "auto telling machine" where I get my cash from the bank. Have I missed something?
The definition of "codsmology" is derived from the Latin "codswhallop" which means a pile of cow S**t" hence codsmology.
Hope this helps.
The conversation on this is persently on against the mainstream, "what happened to the energy". Join Us.
Cheers Lyndon

ZaphodBeeblebrox
2004-Jul-27, 10:34 PM
Watch the Language, this is a Child-Friendly Board.

The BA frowns upon the use of Asterisks to blot out letters in questionable words.

lyndonashmore
2004-Jul-28, 08:13 PM
You are entirely correct. It will not happen again

ZaphodBeeblebrox
2004-Jul-28, 09:01 PM
You are entirely correct. It will not happen again

Good

Thank you.

tjm220
2004-Jul-28, 09:23 PM
The trouble is that apart from trying to bring cosmology down to Earth by showing that the BB theory is a load of codsmology...

What is the definition of "codsmology" and why is your comment not in ATM?
What is ATM? to me it is "auto telling machine" where I get my cash from the bank. Have I missed something?
The definition of "codsmology" is derived from the Latin "codswhallop" which means a pile of cow [deleted] hence codsmology.
Hope this helps...
The conversation on this is persently on against the mainstream, "what happened to the energy". Join Us.
Cheers Lyndon

Thanks for the definition.