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Old 06-04-2008, 07:46 PM #1
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Default Why is the sky blue? (Answer)

Ok, been lurking around wikipedia
and got to sum all this up,
it is VERY interesting if you ask me, and has a simple explanation. If you want to know why we perceive the sky as blue, read on!


(Note: The important processes in the atmosphere (Rayleigh scattering and Mie scattering) are elastic. No energy transformation results, only a change in the spatial distribution of the radiation.)

Simpler explanation (better one (IMO) below): When one looks at the sky during the day, rather than seeing the black of space, one sees light from Rayleigh scattering off the air. Rayleigh scattering is inversely proportional to the fourth power of wavelength, which means that the shorter wavelength of blue light will scatter more than the longer wavelengths of green and red light. This gives the sky a blue appearance. Conversely, when one looks towards the sun, one sees the colors that were not scattered away - the longer wavelengths such as red and yellow light. When the sun is near the horizon, the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. Accordingly, the gradient from a red-yellow sun to the blue sky is considerably sharper at sunrise and sunset.


Better (IMO) explanation:

The sunlit sky appears blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. The result is that the human eye perceives blue when looking toward parts of the sky other than the sun. Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth's surface, so that the light's path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red.



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Old 06-04-2008, 07:53 PM #2
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Default Re: Why is the sky blue? (Answer)

Despite all the fancy physics, I believe the scattering of blue is due to the oxygen. *If you ever look at liquid oxygen, it has a slightly blue hue. *Then when you take the large volume of oxygen between you and the sun, all that oxygen gives a blue hue to the sky. *At least that's what my chem prof in college explained.
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Old 06-04-2008, 08:00 PM #3
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by rkcstr
Despite all the fancy physics, I believe the scattering of blue is due to the oxygen. *If you ever look at liquid oxygen, it has a slightly blue hue. *Then when you take the large volume of oxygen between you and the sun, all that oxygen gives a blue hue to the sky. *At least that's what my chem prof in college explained.
But one of air's components is Oxygen. So Rayleigh effect IS produced on oxygen.

(I've never seen liquid oxygen, but I'd like to)
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Old 06-04-2008, 08:13 PM #4
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Default Re: Why is the sky blue? (Answer)

Yes, oxygen IS a component of air

I just thought I'd clarify that when they talk about scattering off "air", they're most likely meaning the oxygen within the air that causes the blue.
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Old 06-04-2008, 08:25 PM #5
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by rkcstr
Yes, oxygen IS a component of air *

I just thought I'd clarify that when they talk about scattering off "air", they're most likely meaning the oxygen within the air that causes the blue.
I thought the same thing when I was reading it: "Why are they just calling it "AIR", I'm at Wikipedia!"

But well, if you want me to I can modify it...
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Old 06-04-2008, 10:24 PM #6
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Default Re: Why is the sky blue? (Answer)

I'd rather think they mean nitrogen, after all it's nearly 80% of the air. : But come to think of it, Rayleigh scattering refers to scattering by particles comparable in size with the wavelength.Since both the O[sub]2[/sub] and N[sub]2[/sub] molecules are (probably) rounghly the same size, Rayleigh scattering works with both.Just don't call air oxygen.Oxygen is only the active "ingredient" when it comes to breathing, but when it comes to other phenomena , you really must take nitrogen into consideration. :P

Mie scattering takes place when the particle is way bigger than the wavelength, like fog/smoke particles.Ever wondered why the beam always looks brighter when you look towards the source? --> Mie scattering :P
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Old 06-05-2008, 12:00 AM #7
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by Switch
I'd rather think they mean nitrogen, after all it's nearly 80% of the air. : But come to think of it, Rayleigh scattering refers to scattering by particles comparable in size with the wavelength.Since both the O[sub]2[/sub] and N[sub]2[/sub] molecules are (probably) rounghly the same size, Rayleigh scattering works with both.Just don't call air oxygen.Oxygen is only the active "ingredient" when it comes to breathing, but when it comes to other phenomena , you really must take nitrogen into consideration. :P

Mie scattering takes place when the particle is way bigger than the wavelength, like fog/smoke particles.Ever wondered why the beam always looks brighter when you look towards the source? --> Mie scattering :P
Ok, to calculate the weight of one molecule we have to:

O[sub]2[/sub] = 2*O = 2*(16g) = 32g --> 1 mol

32g --- 6,67*10^23 molecules
x ------ 1 molecule

x = 4.79*10^-23 grams.

How the hell can i know it's size?

It has to be at least the same size as 400 to 480nm so we can see Rayleigh with blue colour, hasn't it?
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Old 06-05-2008, 02:40 AM #8
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Default Re: Why is the sky blue? (Answer)

The sky is blue because oxygen is blue (no really, it is). ;D
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Old 06-05-2008, 03:03 AM #9
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by mikewitt
The sky is blue because oxygen is blue (no really, it is). *;D
a single element by itself doesnt have colour

maybe a high amount of it does...

are you 100% sure of this?
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Old 06-05-2008, 03:13 AM #10
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by nikokapo
[quote author=mikewitt link=1212608788/0#7 date=1212633638]The sky is blue because oxygen is blue (no really, it is). ;D
a single element by itself doesnt have colour

maybe a high amount of it does...

are you 100% sure of this?[/quote]

uhh carbon is black :P

btw oxygen really is blue
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Old 06-05-2008, 03:24 AM #11
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by MarioMaster
[quote author=nikokapo link=1212608788/0#8 date=1212634987][quote author=mikewitt link=1212608788/0#7 date=1212633638]The sky is blue because oxygen is blue (no really, it is). *;D
a single element by itself doesnt have colour

maybe a high amount of it does...

are you 100% sure of this?[/quote]

uhh carbon is black *:P

btw oxygen really is blue[/quote]


how is that possible?

isnt colour something we perceive because of the reflection/absorption of different wavelengths on objects?
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Old 06-05-2008, 03:27 AM #12
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Default Re: Why is the sky blue? (Answer)

actually, Rayleigh scattering has nothing to do with whether there is oxygen or nitrogen in the air. it may be true that oxygen has a slightly blue tint to it, but the predominant reason for the sky being blue is rayleigh scattering off the molecules of air. In fact there the sky would actually be more purple if not for several limiting factors. For one, our eyes are incredibly more sensitive to blue than they are to violet so this makes the sky appear more blue. Second, the atmosphere absorbs most of the very violet and ultraviolet light. The sun also radiates more blue than it does the shorter wavelengths. After all these limiting factors and other more subtle things, the sky appears blue.
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Old 06-05-2008, 03:36 AM #13
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Default Re: Why is the sky blue? (Answer)

water is also blue
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Old 06-05-2008, 11:04 AM #14
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by climbak
actually, Rayleigh scattering has nothing to do with whether there is oxygen or nitrogen in the air. it may be true that oxygen has a slightly blue tint to it, but the predominant reason for the sky being blue is rayleigh scattering off the molecules of air. In fact there the sky would actually be more purple if not for several limiting factors. For one, our eyes are incredibly more sensitive to blue than they are to violet so this makes the sky appear more blue. Second, the atmosphere absorbs most of the very violet and ultraviolet light. The sun also radiates more blue than it does the shorter wavelengths. After all these limiting factors and other more subtle things, the sky appears blue.
But aren't the molecules of air actually N[sub]2[/sub] and O[sub]2[/sub] molecules? At least 99% of it. :-/

Btw, I think this is what I meant with the beam apearing brigther when you look down the beam.
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Old 06-05-2008, 02:46 PM #15
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Default Re: Why is the sky blue? (Answer)

Quote:
Originally Posted by nikokapo
[quote author=mikewitt link=1212608788/0#7 date=1212633638]The sky is blue because oxygen is blue (no really, it is). ;D
a single element by itself doesnt have colour

maybe a high amount of it does...

are you 100% sure of this?[/quote]
Yes, I am.

Oxygen, while only ~20% of "air" is blue, whereas nitrogen is a greenish-brown, but multiple orders of magnitude less color-saturated than oxygen. We see the oxygen because it is much more saturated. If you've ever seen liquid oxygen, you know it's blue, and not just like water is blue (and yes, water too is blue), it's REALLY blue.

This became a week long discussion in our physics class (it's fun to distract the teacher :) which ended with us making liquid oxygen. You do it by inflating a balloon with your lungs, then dipping the inflated balloon in liquid nitrogen (which, by the way looks clear). You see a deep bluish liquid form in the bottom of the balloon; that's liquid oxygen.
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Old 06-05-2008, 03:03 PM #16
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Default Re: Why is the sky blue? (Answer)

Meh, "the sky is blue because oxygen is blue" : That's no answer.....why is oxygen blue? :P
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