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Old 06-05-2008, 07:09 PM #33
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Default Re: Why is the sky blue? (Answer)

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Originally Posted by nikokapo
"In astrophysics and cosmology, dark matter is a hypothetical form of matter that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter."

Good enough, I have to go finish a History essay
Haha, you've still got class :P


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Old 06-05-2008, 07:41 PM #34
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Default Re: Why is the sky blue? (Answer)

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Originally Posted by mikewitt
You asked how an object could have color in a vacuum. The object to which you are surely referring is composed of molecules.
Ah I think I understand where you go wrong. The absorption/reflection of radiation by an normal sized object is not called rayleigh scattering. Rayleigh scattering occurs when radiation hits free airborne particles with a size < the wavelength of that radiation:



Extensive explanation (better then wiki ihmo) here:

http://hyperphysics.phy-astr.gsu.edu...os/blusky.html

Bottom line: no air, no rayleigh scattering.
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Old 06-05-2008, 07:53 PM #35
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by Petrovski
[quote author=mikewitt link=1212608788/24#29 date=1212691851]
You asked how an object could have color in a vacuum. The object to which you are surely referring is composed of molecules.
Ah I think I understand where you go wrong. The absorption/reflection of radiation by an normal sized object is not called rayleigh scattering. Rayleigh scattering occurs when radiation hits free airborne particles with a size < the wavelength of that radiation:



Extensive explanation (better then wiki ihmo) here:

http://hyperphysics.phy-astr.gsu.edu...os/blusky.html

Bottom line: no air, no rayleigh scattering.
[/quote]
I see.

Although, why wouldn't mie scattering still occur at apogee?
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Old 06-05-2008, 08:37 PM #36
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Default Re: Why is the sky blue? (Answer)

FINALLY i understand the difference between them (besides what the theory about particle size said). That pic was the key


How can i differentiate between Tyndall and Rayleigh Effect? is it because tyndall is used on colloids only?
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Old 06-06-2008, 02:44 PM #37
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Default Re: Why is the sky blue? (Answer)

The Tyndall-effect and Rayleigh scattering both describe the same natural phenomenon. The Tyndall-effect can be seen as a subcategory of Rayleigh scattering I suppose, it describes the scattering of white light which will occur if the particles are large enough.
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Old 06-07-2008, 12:08 AM #38
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Default Re: Why is the sky blue? (Answer)

Quote:
Objects are not inherently one color or another
Yes they are! Grass is green because it mostly reflects green, and it will always reflect green as long as it's green.Every object that reflects green will be green.I don't know what you mean by "inherently" , but "color" is the property of an object to reflect some wavlengths(in our perceivable spectrum) more than others.And objects "inherently" reflect some wavelengths more than others. :P

I was saying that it's not the same with oxygen, because it does reflect blue, but it doesn't absorb nor filter red, green and yellow like regular solid objects in our surroundings would, it just lets them pass through with less scatter and less reflection.

Quote:
the sky would look UV.
Lol, you mean black. :P

Quote:
If rayleigh scatting would be an explanation for the color of an object, that object would then lose its color in a deep vacuum. After all, rayleigh scatting needs particles < wavelength of light to occur.
It is the explanation of the color of an object(not the general explanation of colors of all objects), if you like refering to the mass of air in the atmosphere like that.That "object" wouldn't loose it's color in a vacuum because if the vacuum would contain that object , it wouldn't be a vacuum anymore, it would have at least one air molecule to scatter light. :
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And what atoms or molecules do you expect to find in a deep vacuum, like for instance space?
Space isn't a vacuum , it has lots of particles and space dust and radiation going through, besides dark matter.Of course a perfect void wouldn't have color, it would just be as transparent as possible.
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Old 06-07-2008, 06:34 AM #39
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Default Re: Why is the sky blue? (Answer)

There's no such thing as a perfect vacuum, there's always something there, just not much.

To answer some earlier questions, molecules, such as an O2, N2, or H2O molecule, are 2 orders of magnitude smaller than visible light. Atoms are generally measured in angstroms (angstrom = .1 nanometer), and visible light is 400 nm. 1nm to 400nm is 2 orders of magnitude.

Also, reflected light is not the only thing that influences a thing's "color". It is the light leaving the object, but that light can be from multiple things. The other most popular is that we see the compliment to a color that is absorbed by electronic transitions within a material, ie it absorbs one color so we see the opposite color, not necessarily that it is reflecting the other color any more strongly.

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Old 06-07-2008, 08:54 AM #40
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Default Re: Why is the sky blue? (Answer)

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Originally Posted by Switch
It is the explanation of the color of an object(not the general explanation of colors of all objects), if you like refering to the mass of air in the atmosphere like that.That "object" wouldn't loose it's color in a vacuum because if the vacuum would contain that object , it wouldn't be a vacuum anymore, it would have at least one air molecule to scatter light. :
Who says a vacuum cannot contains objects? A vacuum only means that there is no air. And without air (or more precise: the particles of the gases in the air) rayleigh scattering will not occur. I could place a 100% pure gold sample in a vacuum, so no "air molecules" (air is not an element) or at least very few particles would be present when compared to earth's atmosphere. So the rayleigh scattering would be absent or at least down to a minimum, yet the gold would still have all of its color in the vacuum.

What exactly do you mean with the quote that rayleigh scatting is an explanation of the color of an object? Do you mean the color of the scattering is particle/molecule dependent?

Quote:
Space isn't a vacuum , it has lots of particles and space dust and radiation going through, besides dark matter.Of course a perfect void wouldn't have color, it would just be as transparent as possible.
A perfect vacuum does not exist, but space is the closest thing to it AFAIK. Again, a void is not the same as a vacuum. A void is the absence of everything, a vacuum is the absence of air.
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Old 06-07-2008, 09:01 AM #41
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Default Re: Why is the sky blue? (Answer)

We can get vacuum as good as space here on earth. Ultra-high vacuum, UHV, processing chambers are routinely operated in the 10^-12 Torr range. This is on par with numbers I have heard quoted from text books as being the pressure in "outer space", but we can't get to those places. Plus, having an object in a vacuum almost guarantees that the pressure will be at least somewhat higher. EVERYTHING has a vapor pressure, and when you're in the 10^-12 Torr range, the vapor pressure of everything is significant. At these levels the stainless steel chambers can have a significant vapor pressure. The cargo bay of the space shuttle can reach pressures no where near this low when it's open, because the shuttle itself is constantly outgassing and has a vapor pressure of its own as well.
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Old 06-07-2008, 10:34 AM #42
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Default Re: Why is the sky blue? (Answer)

Yeah... no such thing as a perfect vacuum. Gotta love these theoretical discussions. :P
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Old 06-07-2008, 06:16 PM #43
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Default Re: Why is the sky blue? (Answer)

I read the whole discussion.

WOW.

Please go on!
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Old 06-09-2008, 11:10 AM #44
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Default Re: Why is the sky blue? (Answer)

Quote:
A perfect vacuum does not exist, but space is the closest thing to it AFAIK. Again, a void is not the same as a vacuum. A void is the absence of everything, a vacuum is the absence of air.
It would still be really transparent. Of course, it wouldn't be a void anymore if light was shining through it.
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Old 06-09-2008, 04:33 PM #45
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Default Re: Why is the sky blue? (Answer)

In a perfect void transparency would also be absent? :P
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Old 06-09-2008, 08:04 PM #46
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Default Re: Why is the sky blue? (Answer)

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In a perfect void transparency would also be absent? *:P
Transparency is a characteristic.A concept.It's not matter, nor energy , nor nothing that exists beyond the human mind. :P
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Old 07-01-2008, 10:57 AM #47
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Default Re: Why is the sky blue? (Answer)

there also lots of cloud on the horizon, or dust if your in the desers, and definitly salt if looking out to sea.
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Old 07-05-2008, 09:37 AM #48
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Default Re: Why is the sky blue? (Answer)

I was told the O3 (ozone) is the predominent gas the colors the sky. Oxygen turns to ozone when it absorbs high energy shortwave energy. O2->O3. DeBroegle theorized that a atom or monacule can emit 18 times the light it absorbs. Go figure?

We will get skin cancer if the ozone depleates. And the earths magnetic field helps bounce of gamma rays and such to.

There is no simple answer.
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