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Author Topic: why is the sky blue  (Read 5818 times)

Offline super sci fi zipidoo dahh

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why is the sky blue
« on: 18/05/2007 16:29:22 »
I know this sounds like an absolutly stupid question which I believed to know the answer for years, however now so we are having dispuits! someone reply so I can end this disscussion!
« Last Edit: 26/05/2007 00:06:03 by ukmicky »


 

paul.fr

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Re: why is the sky blue
« Reply #1 on: 18/05/2007 16:36:56 »
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The Really Deep explanation of why the sky is blue is hard. Albert Einstein wrote, in 1911, that Relativity had to be used to fully understand why the sky is blue.

But let's do the easier explanation.

It begins with understanding something called "scattering" of light. "Scattering" means that the light hits some kind of particle, is absorbed, and then, is re-emitted. The light might get changed between being absorbed and emitted, changing in (say) brightness, or angle at which it is emitted, or even colour.

It was John William Strutt, 3rd Baron Rayleigh, also known as Lord Rayleigh, who looked specifically at light being scattered by very small particles, such as molecules. Lord Rayleigh was an intellectual heavyweight, winning the Nobel Prize in 1904. But it was earlier than that, back in the 1870s, that he gave us the now-famous "Rayleigh Scattering", which explains why the sky is blue. Rayleigh Scattering works when the particle is less than about 3% of the wavelength of light. Molecules of nitrogen and oxygen are a few thousand times smaller than the wavelength of light, so the "Rayleigh Scattering Effect" is definitely happening here.

The atmosphere is made from various gases - about 77% nitrogen, 21% oxygen, 1% argon, with several other gases (including carbon dioxide) making up the remaining 1%. These gases are, in turn, made up of many molecules. These molecules are very small, and very far from each other. For the incoming sunlight in our atmosphere, it's as though it travels in a vacuum, misses many molecules, and then hits a molecule and gets "scattered". This scattered light then continues travelling in a vacuum until it hits another molecule in our atmosphere. Each time the light hits a molecule, there's a time delay, during which time the light is absorbed, and then re-emitted.

Now here's the essence of Rayleigh Scattering. First, the re-emitted light does not come out of the molecule in the direction from which it came - instead, it comes out mostly at right angles to its original direction. Second, the "right-angle bending effect" is much stronger for blue light than for red light, so the emitted light is predominately blue. The red light goes mostly straight ahead, and then back out of the atmosphere into deep space - and so you never see it. Third, your eyeball will register seeing a colour only if some light of that colour lands inside your eyeball.

So white light will hit a gas molecule, but bluish light will come out - and at right angles to the original direction. This light will then travel in a straight line until it hits another molecule, and the whole absorb/emit/right angle/blue thing happens again. So the light that came from the sun will hit a gas molecule high in the atmosphere, and then will stutter in a zig-zag fashion down towards the ground. Each time the light hits a molecule, the light comes out mostly at right angles to its previous direction of travel - and each time, it's more blue in colour. Eventually, some light that has been through thousands of these absorb/emit situations with thousands of molecules will land in your eyeball - and then, and only then, do you get the impression of seeing blue.

There you have it - the sky is blue is because of Rayleigh Scattering by the nitrogen and oxygen molecules in the atmosphere. And the ocean is blue because of Rayleigh Scattering by the water molecules that make up the water, as well as reflecting the sky

 

Offline kdlynn

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Re: why is the sky blue
« Reply #2 on: 18/05/2007 17:56:56 »
and that was the easy answer!
 

Offline lightarrow

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Re: why is the sky blue
« Reply #3 on: 18/05/2007 19:11:52 »
and that was the easy answer!

Do you want an easy answer? here you have:

Take two laser-pens, one with red light beam, another with green light beam; the last are sold in electronic shops, as useful pointers for astronomy. They are useful because you can point a star, in the night, and people will easily see the beam. It would be much more difficult with a red light laser-pen.

This is because green light is scattered from air molecules more than red light, and this, in turn, because green light has a lower wavelenght than red light.

So, red light passes through air easier than the green one, and green easier than blue (blue light has an even higher wavelenght).

Before you ask why astronomers don't use blue lasers, I tell you now: they are not sold (too much expensive).
« Last Edit: 18/05/2007 19:19:35 by lightarrow »
 

Offline ukmicky

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Re: why is the sky blue
« Reply #4 on: 18/05/2007 21:29:18 »
It is possible for the sky to go green during the very last seconds of a sunset.
 

paul.fr

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Re: why is the sky blue
« Reply #5 on: 18/05/2007 21:52:28 »
Yes, but i am not sure why. during the Aurora Borealis the sky can turn green and i think green flashes in the sky are due to iron meterites breaking up in the atmosphere.
 

Offline super sci fi zipidoo dahh

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Re: why is the sky blue
« Reply #6 on: 19/05/2007 03:31:59 »
thanks for your replies!!
 

Offline lightarrow

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Re: why is the sky blue
« Reply #7 on: 20/05/2007 11:14:49 »
It is possible for the sky to go green during the very last seconds of a sunset.
Yes, is due to light refraction from the athmosphere. Shorter wavelenght are refracted more, so when the sun disappears under the horizon, the higher portion of the disk seems still there, but of the shorter wavelenghts colours. Since blue colour is scattered, the last colour you see is green. At the poles, the effect can last half an hour.
 

lyner

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Re: why is the sky blue
« Reply #8 on: 22/05/2007 11:05:58 »
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Since blue colour is scattered, the last colour you see is green.
So what about the red - the longest wavelength?
There will be less and less energy from the shorter wavelengths - as  the light travels through a longer and longer path through the atmosphere. The green Sun is possibly a psycho-visual (!!!) effect.
All things being equal, you would expect just to see the setting Sun getting redder and redder. BUT, I think that you may well perceive the Sun's disc to be green because you will see a lot of, predominantly, red clouds around the Western horizon. You brain is constantly trying to do its best to 'see' colours correctly. It has to assume that the overall scene is 'grey' and, therefore, will compensate by 'reducing the red channel'. This will make the  red + green Sun  (which should look yellow) look more green, albeit a very subtle shade of green.
Remember, the colours of most objects we see are not very saturated (pure) but are diluted with traces of all wavelengths. For instance,white  things do not appear blue to us when illuminated just by 'blue'  sky i.e. in the shade. The blue sky is not really very blue at all - it has quite a lot of all wavelengths and is desaturated. Our colour discrimination is pretty good - we can detect changes of much less than 1% , which is why we need at least 256 levels or 8bits for the three channels in  digital photographs to avoid seeing unpleasant contours in large areas of almost the same colour.
 

Offline lightarrow

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Re: why is the sky blue
« Reply #9 on: 23/05/2007 17:20:49 »
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Since blue colour is scattered, the last colour you see is green.
So what about the red - the longest wavelength?
Red goes quite straight, so you don't see it anylonger when the sun is under the horizon. But, you still see a green light, in some places under some special conditions, because shorter wavelenght don't go quite straight, but are bent (more precisely: are bent more than red) so they goes down to your eyes and you can still see them.
Quote
There will be less and less energy from the shorter wavelengths - as  the light travels through a longer and longer path through the atmosphere. The green Sun is possibly a psycho-visual (!!!) effect.
I have a book where this effect is explained: "The flying circus of physics. With answers" - Jearl Walker; question n. 5.16: "The green flash". Very nice book! Also very simple. It says that this effect has been studied deeply and pictures of the green flash has actually been taken with cameras.
See for example:
http://en.wikipedia.org/wiki/Green_flash
http://mintaka.sdsu.edu/GF/explain/obs_colors.html
http://spaceweather.com/swpod2006/31jan06/zinkova.jpg
Here the effect is explained with drawings:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html
« Last Edit: 23/05/2007 17:34:39 by lightarrow »
 

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Re: why is the sky blue
« Reply #9 on: 23/05/2007 17:20:49 »

 

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