# Sky
**URL:** https://boards.straightdope.com/t/sky/10554
**Category:** Factual Questions
**Created:** [April 8, 2000, 11:14pm UTC](https://boards.straightdope.com/t/sky/10554 "2000-04-08T23:14:00Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Audi](https://boards.straightdope.com/u/Audi)
#### Post date: [April 8, 2000, 11:14pm UTC](https://boards.straightdope.com/t/sky/10554/1 "2000-04-08T23:14:00Z")
In outer space the color is black, but when look up all you see is blue and clouds. Why is this?
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### Author:  [@WallyM7](https://boards.straightdope.com/u/WallyM7)
#### Post date: [April 8, 2000, 11:20pm UTC](https://boards.straightdope.com/t/sky/10554/2 "2000-04-08T23:20:00Z")
Hi, Audi. How old are you?
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### Author:  [@Jophiel](https://boards.straightdope.com/u/Jophiel)
#### Post date: [April 8, 2000, 11:40pm UTC](https://boards.straightdope.com/t/sky/10554/3 "2000-04-08T23:40:00Z")
On the off chance that this is a legit post:
The atmosphere is full of water particles. As the sun’s light shines through the air, it gets scattered by the water molecules. the blue wavelengths are the strongest and so they are the most easily seen. During a sunrise or sunset though, the sun’s light has to pass through more air to get to us and the blue wavelengths are mainly scattered away, leaving only the red wavelengths to be seen.
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“I guess one person can make a difference, although most of the time they probably shouldn’t.”
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### Author:  [@Demo](https://boards.straightdope.com/u/Demo)
#### Post date: [April 9, 2000, 1:36am UTC](https://boards.straightdope.com/t/sky/10554/4 "2000-04-09T01:36:00Z")
Also known as **refraction**. The black you witness in space is simply the abscence of this phenomenon.
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### Author:  [@aha](https://boards.straightdope.com/u/aha)
#### Post date: [April 9, 2000, 1:42am UTC](https://boards.straightdope.com/t/sky/10554/5 "2000-04-09T01:42:00Z")
> [@](#):
>
> Originally posted by Audi:
> **In outer space the color is black, but when look up all you see is blue and clouds. Why is this?**
Actually black is the _absence_ of color sonny. 🙂
…
I would rather have a bottle in front of me than a frontal lobotomy.
Yours truly,
aha
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Yours truly,
aha
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [April 9, 2000, 1:43am UTC](https://boards.straightdope.com/t/sky/10554/6 "2000-04-09T01:43:00Z")
It’s scattering, not refraction. Refraction only bends a beam of light, like a clear piece of glass. It’s the various dust particles which causes scattering and make the sky blue. You can say “Rayleigh scattering” if you want to sound more scientific.
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### Author:  [@m3](https://boards.straightdope.com/u/m3)
#### Post date: [April 9, 2000, 1:52am UTC](https://boards.straightdope.com/t/sky/10554/7 "2000-04-09T01:52:00Z")
Wha’s “stronger” about blue light? I read that it was scattered because of its wavelength.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [April 9, 2000, 3:36am UTC](https://boards.straightdope.com/t/sky/10554/8 "2000-04-09T03:36:00Z")
Basically, waves are not affected by things much smaller than its wavelength. If you think abot ripples on a pond, the wavelength (distance from one peak to the next peak) is, say, a couple of inches. A 1/2 diameter rod sticking out of the water doesn’t do much, but a 3-inch diameter rod will reflect/scatter the wave.
Or, to put it another way, if you have a 1-inch rod sticking out of the water, a 1/2-inch wavelength wave will get scattered much more than a 2-inch wavelength wave. The same thing happens in the sky - minute dust particles have sizes comparable to the wavelength of light, so shorter wavelength light (blue light) is scattered more than longer wavelength light (red light).
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### Author:  [@m3](https://boards.straightdope.com/u/m3)
#### Post date: [April 9, 2000, 3:54am UTC](https://boards.straightdope.com/t/sky/10554/9 "2000-04-09T03:54:00Z")
What about the sun set? If a few particles make the sky blue wouldn’t more of them make the sky bluer rather than making the sky reddish? I remember being confused by this when I first learned this, maybe someone can clear it up.
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### Author:  [@Jophiel](https://boards.straightdope.com/u/Jophiel)
#### Post date: [April 9, 2000, 5:37am UTC](https://boards.straightdope.com/t/sky/10554/10 "2000-04-09T05:37:00Z")
_ahem_ As I just said…
_During a sunrise or sunset though, the sun’s light has to pass through more air to get to us and the blue wavelengths are mainly scattered away, leaving only the red wavelengths to be seen._
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“I guess one person can make a difference, although most of the time they probably shouldn’t.”
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### Author:  [@matt\_mcl](https://boards.straightdope.com/u/matt_mcl)
#### Post date: [April 9, 2000, 4:29pm UTC](https://boards.straightdope.com/t/sky/10554/11 "2000-04-09T16:29:00Z")
This is a perfectly legitimate question and I’m startled that you all would react in such a way.
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### Author:  [@Jophiel](https://boards.straightdope.com/u/Jophiel)
#### Post date: [April 9, 2000, 6:48pm UTC](https://boards.straightdope.com/t/sky/10554/12 "2000-04-09T18:48:00Z")
“You all” who? Only Wally didn’t have something useful to add to the OP. I thought we answered it fairly well.
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“I guess one person can make a difference, although most of the time they probably shouldn’t.”
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### Author:  [@OgreFade](https://boards.straightdope.com/u/OgreFade)
#### Post date: [April 9, 2000, 7:46pm UTC](https://boards.straightdope.com/t/sky/10554/13 "2000-04-09T19:46:00Z")
actually the most abundant wave lenght happens to be green. Thats where the peak of our vision is.
However the sky is blue because of refraction, and due to the curvature of the earth.
Thats almost the same reason for mirages and rainbows. look in a physics book for about 10 minutes you’ll find it. In more detail. THey’ll give you equations i can’t remember.
The angle of refraction changes in a sunset. So you see higher wavelength colors… Red blah blah… its less of a bend. Its still bending though.
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### Author:  [@Demo](https://boards.straightdope.com/u/Demo)
#### Post date: [April 9, 2000, 11:59pm UTC](https://boards.straightdope.com/t/sky/10554/14 "2000-04-09T23:59:00Z")
> [@](#):
>
> Originally posted by scr4:
> \*\*It’s scattering, not refraction. Refraction only bends a beam of light, like a clear piece of glass. It’s the various dust particles which causes scattering and make the sky blue. You can say “Rayleigh scattering” if you want to sound more scientific. \*\*
Whoops! Thanks scr4. 😉
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [April 10, 2000, 6:37am UTC](https://boards.straightdope.com/t/sky/10554/15 "2000-04-10T06:37:00Z")
> [@](#):
>
> However the sky is blue because of refraction, and due to the curvature of the earth. Thats almost the same reason for mirages and rainbows.
Again: the blue color of the sky has nothing to do with refraction. The light is scattered by particles in the atmosphere.
Refraction is what happens when light encounters a boundary between different materials, or different densities of the same material, such as air to glass/water, or cool air to hot air (think mirages). Refraction is also wavelength dependent and can extract certain colors from white light. This is how prisms and rainbows make color. Also, the “green flash” is due to refraction. This is because the green light refracts more than the red, and so the “green sun” appears higher in the sky than the “red sun,” and the green sun is the last to set. (Actually blue sun sets even later, but there’s not much blue light in light from a sunset.)
However, the effect we’re talking about for the green flash is less than a degree. There’s no way you can arrange the atmosphere - or any lens system - so that it refracts blue light towards every single observer on the ground from every possible angle.
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### Author:  [@OgreFade](https://boards.straightdope.com/u/OgreFade)
#### Post date: [April 10, 2000, 6:56am UTC](https://boards.straightdope.com/t/sky/10554/16 "2000-04-10T06:56:00Z")
you said that air was good enough to refract the light… odd… I never claimed to be the expert… if you read anywhere… i’m the biggest loser here. I’m sorry I’m WRONG…
Enjoy …sorry
I lied to you i guess.
Have a good night… screw these boards
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### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [April 12, 2000, 5:32am UTC](https://boards.straightdope.com/t/sky/10554/17 "2000-04-12T05:32:00Z")
This is all well and good, but I took a physics course at Hopkins where the question was proposed by the professor:
All this talk about scattering, etc., still does not preclude the sky from being violet. But, the professor would not comment on this.
a) So, why isn’t the sky violet? Isn’t it’s wavelength even less than blue’s? (True!)
b) Also, if scattering is the deal: Is it meant to say that the other colors are scattered outward from Earth? If so, why don’t these colors appear in photos of Earth from space? i.e: The earth should not be a blue marble with all this scattering.
Everything I’ve read here is just the rhetoric from rote memory as to what we were forced to believe was the whole story…as a simple answer.
I saw a demo in college, also, where white light was shown through a tank comtaining some aqueous solution. From the side, the light appeared blue, but head-on, the light appeared yellow. This was attempting to explain why the sun doesn’t appear blue. But, I still don’t get what this is all about.
Anyone care to dive deeper into this subject?
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“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
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### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [April 11, 2000, 7:37pm UTC](https://boards.straightdope.com/t/sky/10554/18 "2000-04-11T19:37:00Z")
> [@](#):
>
> Originally posted by OgreFade:
> **actually the most abundant wave lenght happens to be green. Thats where the peak of our vision is…**
Hmm, yet another mystery of light! In high school, I was taught that the eye is most sensitive (responsive) to the green frequency because it is in the middle of the visible spectrum. OK, but not really substantial proof.
In college, the same Hopkins physics prof as I mentioned above said that the eye is most sensitive (responsive) to yellow light because the sun is a yellow star, and our eye as “evolved” to respond to this frequency.
Do either theories hold any water? As for the median frequency of visible light, why should this matter? Also, about the constant exposure to yellow from the sun, couldn’t I also argue that the eye should be **least** sensitive to yellow? The eye tends to lose sensitivity to a particular wavelength(s) when over-exposed…the eye fatigues, in a sense. Or so, this is how I was taught about why the eye “sees” an after-image after staring at an object.
Violet skies? Fatigued eyes?
Anyone dare to give a WAG a try???
* * *
“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
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### Author:  [@OgreFade](https://boards.straightdope.com/u/OgreFade)
#### Post date: [April 11, 2000, 7:40pm UTC](https://boards.straightdope.com/t/sky/10554/19 "2000-04-11T19:40:00Z")
actually I’m right about the green. He’s wrong. Hehe, or my prof in hs physics was. he showed us visible litght spectrums and neato stuff like that… i could be wrong… but i’m certain i remember the info given me…
the prof could have been wrong.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [April 12, 2000, 2:03am UTC](https://boards.straightdope.com/t/sky/10554/20 "2000-04-12T02:03:00Z")
> [@](#):
>
> a) So, why isn’t the sky violet? Isn’t it’s wavelength even less than blue’s? (True!)
I think for the human eye to see violet, it has to be a pretty pure violet light. The ‘filtering’ effect is not quite strong enough. Also, there is a lot more blue light in sunlight to begin with.
> [@](#):
>
> b) Also, if scattering is the deal: Is it meant to say that the other colors are scattered outward from Earth?
No, it means that when sunlight passes through the atmosphere, blue light gets scattered in every direction, while green and red light pass right through unaffected. So blue light get scattered both up and down, which is why the earth looks blue from space. The red and green light don’t get scattered at all (or not much), and hit the surface of the earth.
> [@](#):
>
> I saw a demo in college, also, where white light was shown through a tank comtaining some aqueous solution. From the side, the light appeared blue, but head-on, the light appeared yellow. This was attempting to explain why the sun doesn’t appear blue. But, I still don’t get what this is all about.
If the solution was very pure and contained no particles to scatter light, what would you see? You wouldn’t see anything from the side, just a tank of liquid. The beam that comes out the end is the same color that went in the other end - white. Now, what happens if the solution contained big particles, like sand? All colors of light get scattered, so looking at the tank from the side, you see white light coming at you. This can happen with particles much smaller than sand. As long as the particles are much larger than the beam of light, all colors are scattered equally and you get white. That’s why milk is white - milk has big blobs of fat floating around.
Now, let’s take a tank that contains even smaller particles. Smaller than microorganisms. So small they are barely visible in the best optical microscopes out there. So small that they are about the same size as the wavelength of light - some larger, some smaller. Waves are usually not affected by things smaller than their wavelength. Blue light can still get scattered, because the wavelength is short. Red light is scattered to a much lesser degree, since the wavelength is long. So, the particles in the tank scatter blue light in all directions. If you looked at the tank from the side, the scattered light is all you see, so you see blue. The red and green light do not get scattered so they pass right through the tank and come out the end. The light that comes out the end contains a lot of red and green but not much blue - i.e. it’s yellow.
> [@](#):
>
> …that the eye is most sensitive (responsive) to yellow light because the sun is a yellow star, and our eye as “evolved” to respond to this frequency…
First you have to understand what “yellow,” “white” and “green” mean. White light is light that is neutral to our eyes, i.e. the light from our sun. Yellow is basically the same as white, with slightly more red (or slightly less blue). For this discussion the difference is negligible.
So what does this white light, the light from our sun, look like? It is a continuous spectrum which has a broad peak in the green. That is, it has a lot of green, less blue and red, even less ultraviolet and infrared, and not much of anything else.
So yes, green light is the most abundant. And yes, our eyes did evolve to be sensitive in this region, because - well, why try to see colors which are poorly illuminated by the sun?
By the way, the other day I found a wonderful book called “Light and Color in the Outdoors” by M. G. J. Minneart. It explains all the optical phenomena you may see in nature. (This is not meant to be a RTFM remark though, keep good questions like these coming on this board…)
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