colour of the sky

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Essah
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colour of the sky

Postby Essah » Sun May 22, 2011 1:16 am UTC

So my knowledge so far:

the sky is blue because of the angle the the sun rays hit the atmosphere causing the light ray to split into spectrum (dunno how to explain) and the colour we see is the blue. when the sun is low though at sunrise and sunset the rays impact at a different much shallower angle and the resulting colour is different nuances of red.

my question is, why is it red and blue, when they are not 2 colours adjacent to each other in the visible light spectrum. you would think that as the angle changes the colour would slowly go from blue into one of the adjacent colours, that is purple and green, rather than red.

I don't know if this is explained well enough.

KingXimana
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Re: colour of the sky

Postby KingXimana » Sun May 22, 2011 2:01 am UTC

The awnser is simply that you where correct. If you ever have watched a sunset/rise you will notice the sun dosnt suddenly change color but it slowly goes from blue through purple and yellow to red.

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Re: colour of the sky

Postby Seraph » Sun May 22, 2011 2:41 am UTC

KingXimana wrote:The awnser is simply that you where correct.

His description sounds like refraction, which means that he isn't correct.

The color of the sky during daylight is an effect of Rayleigh scattering, not refraction. Short wavelength (blue) light gets scattered by the atmosphere more then long wavelength light. The effect is that blue light appears to come from everywhere, but the other colors appear to come from the sun (yellow basically being white light with the blue removed). As the sun starts to set, and the light has to travel through more and more of the atmosphere you eventually reach the point where the short wavelengths never reach you, so what's left shifts towards the red end of the spectrum.

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Re: colour of the sky

Postby cpt » Sun May 22, 2011 3:53 pm UTC

KingXimana wrote:The awnser is simply that you where correct. If you ever have watched a sunset/rise you will notice the sun dosnt suddenly change color but it slowly goes from blue through purple and yellow to red.


At any point, has anybody ever noticed the sun to be purple or blue??

Seraph wrote:His description sounds like refraction, which means that he isn't correct.

The color of the sky during daylight is an effect of Rayleigh scattering, not refraction. Short wavelength (blue) light gets scattered by the atmosphere more then long wavelength light. The effect is that blue light appears to come from everywhere, but the other colors appear to come from the sun (yellow basically being white light with the blue removed). As the sun starts to set, and the light has to travel through more and more of the atmosphere you eventually reach the point where the short wavelengths never reach you, so what's left shifts towards the red end of the spectrum.


This is spot-on. As an addition, to those of you who ask "Then why isn't the sky violet if the shortest wavelengths are scattered most?", it's because of our human eyes being much less sensitive to indigo/violet (purple) light than to blue.

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thoughtfully
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Re: colour of the sky

Postby thoughtfully » Sun May 22, 2011 7:28 pm UTC

To bed pedantic, it isn't exactly correct. The Sun is not white. It isn't nearly hot enough at its surface. Yellow is what you have left over when you remove blue from white, but the Sun still emits plenty of blue anyway, just not enough to fully balance the lower wavelengths.
Last edited by thoughtfully on Sun May 22, 2011 10:36 pm UTC, edited 1 time in total.
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Jakell
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Re: colour of the sky

Postby Jakell » Sun May 22, 2011 9:29 pm UTC

cpt wrote:
KingXimana wrote:The awnser is simply that you where correct. If you ever have watched a sunset/rise you will notice the sun dosnt suddenly change color but it slowly goes from blue through purple and yellow to red.


At any point, has anybody ever noticed the sun to be purple or blue??

I would like to note, that wile the sun never really looks blue or violet, it does on occasion look green.

thoughtfully wrote:To bed pedantic, it isn't exactly correct. The Sun is not white. It isn't nearly hot enough at its surface. Yellow is what you have left over when you remove blue from white, but the Sun still emits plenty of blue anyway, just not enough to fully balance the lower wavelengths.

Of course the sun is white. Since the radiation intensity peaks in the yellow/green part of the spectrum, it is classified as a yellow dwarf, but it gives off enough light across the whole visible spectrum to appear white. This picture of the sun as seen from space illustrates this nicely, while also showing a reflection off the surface of the ocean. Note the distinctly yellow look of the refection, due two twice as much Rayleigh scattering as we normally see.
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Re: colour of the sky

Postby ST47 » Sun May 22, 2011 10:28 pm UTC

Jakell wrote:
thoughtfully wrote:To bed pedantic, it isn't exactly correct. The Sun is not white. It isn't nearly hot enough at its surface. Yellow is what you have left over when you remove blue from white, but the Sun still emits plenty of blue anyway, just not enough to fully balance the lower wavelengths.

Of course the sun is white. Since the radiation intensity peaks in the yellow/green part of the spectrum, it is classified as a yellow dwarf, but it gives off enough light across the whole visible spectrum to appear white. This picture of the sun as seen from space illustrates this nicely, while also showing a reflection off the surface of the ocean. Note the distinctly yellow look of the refection, due two twice as much Rayleigh scattering as we normally see.

I suspect that the color there is much more due to the CCD becoming saturated than it is due to the actual color of the sun. There is enough light coming in at all wavelengths to have the CCD read its highest possible intensity reading, however that doesn't preclude the possibility that the redder frequencies are more intense than the blue and violet frequencies.

Also, to be pedantic, I don't think bedding pedantic is something that makes much sense.

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Re: colour of the sky

Postby Soralin » Sun May 22, 2011 10:54 pm UTC

ST47 wrote:I suspect that the color there is much more due to the CCD becoming saturated than it is due to the actual color of the sun. There is enough light coming in at all wavelengths to have the CCD read its highest possible intensity reading, however that doesn't preclude the possibility that the redder frequencies are more intense than the blue and violet frequencies.

Also, to be pedantic, I don't think bedding pedantic is something that makes much sense.

It's not just the CCD, I've seen other descriptions of the sun as been white when seen from orbit. Also:
http://en.wikipedia.org/wiki/Sun wrote:The Sun's stellar classification, based on spectral class, is G2V, and is informally designated as a yellow dwarf, because its visible radiation is most intense in the yellow-green portion of the spectrum and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.[14][15]

So not just the CCD, but our own eyes see it the same way. :) I've also seen that in explanations of why we never see any green stars (or anything being green-hot): Because any stars which produce light which peaks in the frequency of green light, also produce a ton of light in blue and red as well, so they just end up appearing white to our eyes.

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Re: colour of the sky

Postby thoughtfully » Sun May 22, 2011 11:04 pm UTC

Looks like I get to eat crow here. I googled around, and the whiteness was well corroborated. There are actually a few Java apps that will simulate the perceived color (most just plot the spectrum). This was the nicest one, although Wolfram has one that requires you to download a special viewer.
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Re: colour of the sky

Postby SU3SU2U1 » Mon May 23, 2011 1:38 am UTC

The sky is blue because air is very faintly blue.

Understanding that Rayleigh scattering explains why collections of particles scatter blue light is nice- but the short explanation is simply that air is blue.

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Re: colour of the sky

Postby sikyon » Mon May 23, 2011 2:32 am UTC

SU3SU2U1 wrote:The sky is blue because air is very faintly blue.

Understanding that Rayleigh scattering explains why collections of particles scatter blue light is nice- but the short explanation is simply that air is blue.


That's not right at all. When you say an object is blue, you are saying that the object will absorb all light except blue light, which is reflected. This is the opposite of what happens in the atmosphere. Specifically because the atmosphere is relatively transparent to non blue light does the atmosphere appear blue.

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Re: colour of the sky

Postby SU3SU2U1 » Mon May 23, 2011 2:47 am UTC

sikyon wrote:That's not right at all. When you say an object is blue, you are saying that the object will absorb all light except blue light, which is reflected. This is the opposite of what happens in the atmosphere. Specifically because the atmosphere is relatively transparent to non blue light does the atmosphere appear blue.


Thats not alway what you mean when you say something is blue (or any color).

There are lots of materials that are different in color depending on the angle you look at them, for instance (often because they transmit differently than they reflect). The sky has color in the exact same way a dichroic material (certain types of glass bead, for instance) has color.

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Re: colour of the sky

Postby Essah » Mon May 23, 2011 2:55 am UTC

SU3SU2U1 wrote:The sky is blue because air is very faintly blue.

Understanding that Rayleigh scattering explains why collections of particles scatter blue light is nice- but the short explanation is simply that air is blue.


really doesn't make sense to what I've learned so far. AFAIK everything colour is actually just the colour of the light that it reflects.
so the answer is that the short wavelength being blue gets scattered and fills the sky so it appears blue and with the blue removed the light we see from the sun is yellow being longer wavelength and not scattering. then when the rays travel more the blue and yellow doesn't reach us and the red starts to scatter painting the sun and the sky around the sun (from earth POV) purple-reddish.

is that correctly understood? :D

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Re: colour of the sky

Postby Jakell » Mon May 23, 2011 3:11 am UTC

SU3SU2U1 wrote:There are lots of materials that are different in color depending on the angle you look at them, for instance (often because they transmit differently than they reflect). The sky has color in the exact same way a dichroic material (certain types of glass bead, for instance) has color.

No! Goodness, no. Dichroic materials depend on the interference of wavelengths of light; this is verymuch not what is happening in the blue sky.
Rayleigh Scattering is very easy to show, you can do this experiment by simply adding a small amount of milk to an aquarium tank full of water; shine a flashlight through the water and from the side it looks blue, looking at the light directly through the water and it looks yellowish, reddish if you add a lot of milk.
Because Rayleigh Scattering has a strong dependence on wavelength (1/λ^4), the smaller the wavelength, the more the light gets scattered by the molecules. This is why the sun gets redder as it approaches the horizon - more of the higher frequency light scatters out since the light has to travel through more atmosphere on it's way to us. This is also why fluorescent clothing stays so bright for a long while after the sun sets; the blue, violet, and even some ultraviolet light is still being scattered down onto you for a good deal of time after the sun is down.

SU3SU2U1 wrote:The sky is blue because air is very faintly blue.

And air is not blue, even faintly. The color is almost entirely due to Rayleigh scattering, and a few other modes of scattering. Similarly, the sky becomes redder when the air is filled with more contaminants; the increased levels of pollution have made sunsets particularly lovelier, and the effect is enhanced even further when natural disasters fill the air with smoke particles, such as in forestfires and volcanic eruptions. These have demonstrable effects on the perceived color of the atmosphere, due to the Rayleigh effect.

I would note, though, that while the gasses themselves are colorless, as is liquid nitrogen, liquid oxygen does have a lovely blue color to it. This has no effect on the color of the sky, though.
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Re: colour of the sky

Postby SU3SU2U1 » Mon May 23, 2011 4:21 am UTC

Dichroic materials depend on the interference of wavelengths of light; this is verymuch not what is happening in the blue sky.


Opalescence is a form of dichroism, last I checked, and the blue sky is literally the text-book example. Dichroism just means separating into two colors. At the extremes, the sky transmits in red and scatters in blue.

The "dichroic filter" would perhaps be better thought of as an interference filter- though it uses that interference to produce effectively the same thing, a material reflective in one color and transmitting in another.

Rayleigh Scattering is very easy to show...


Yes, I'm aware, I've taught grad level electricity and magnetism several times. The issue is that saying something scatters blue light preferentially is the same thing as saying something IS blue. Rayleigh scattering is the mechanism that makes air blue.

Also, the milk experiment is not properly Rayleigh scattering, its Tyndall scattering. Rayleigh scattering requires particle sizes much smaller than the wavelength of light, while colloidal particles are comparable in size. You have to treat the scattering in full Mie theory, and its much more complicated. You get the same fourth power of frequency, though, so similar colors/the same dichroic effect.

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Re: colour of the sky

Postby Technical Ben » Mon May 23, 2011 11:15 am UTC

You can do this as a practical test too AFAIR.
Get a large fish tank. Fill it with water. Get a torch, and some talcum powder.
In a dark room, shine the torch through the tank. Start to sprinkle in the talc. It should turn the colour from blue to red to purple etc. As the particles of talc get denser, and the light is scattered along the wavelengths.
I can't find the video or test details online, so it may not work perfectly.
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Re: colour of the sky

Postby sikyon » Tue May 24, 2011 5:18 am UTC

SU3SU2U1 wrote:
sikyon wrote:That's not right at all. When you say an object is blue, you are saying that the object will absorb all light except blue light, which is reflected. This is the opposite of what happens in the atmosphere. Specifically because the atmosphere is relatively transparent to non blue light does the atmosphere appear blue.


Thats not alway what you mean when you say something is blue (or any color).

There are lots of materials that are different in color depending on the angle you look at them, for instance (often because they transmit differently than they reflect). The sky has color in the exact same way a dichroic material (certain types of glass bead, for instance) has color.


The definition of an objects color I gave is the generally used definition, and the only one a layperson would probably be aware of if asked what quantified an object's color. Simply telling someone the sky is blue because the air is blue is incoherent because anyone likely to require such a simple explanation would interpret it incorrectly. Anyone who could interpret it correctly would appreciate a more detailed explanation anyways. I see no context in which your explanation of the sky's color would correctly communicate the concepts accurately and concisely.

Also by defining such materials as having a certain color that depends on angle, I think, is inaccurate and confusing. I don't know what the convention is here but I would not consider a material to scatter light to be intrinsically blue any more than I would consider a white piece of paper put under a blue light to be intrinsically blue. In both cases I would say the object appears blue but I would not quantify the object itself as blue.

You can argue technicalities of correctness all you want but in order to have a good, rational explanation you must consider both context and audience as well as message.

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Re: colour of the sky

Postby Tass » Tue May 24, 2011 6:24 am UTC

On the other hand it is a fine explanation for water. Water actually is blue since it absorbs preferentially in the red end of the spectrum.

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Re: colour of the sky

Postby Jorpho » Fri Jun 03, 2011 3:58 am UTC

I think I've asked this before, but I've forgotten the answer:

If the blue color results because short wavelengths are scattered preferentially, then why isn't the sky purple, since the wavelength of purple is even shorter?

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Re: colour of the sky

Postby ajd007 » Fri Jun 03, 2011 4:26 am UTC

Jorpho wrote:I think I've asked this before, but I've forgotten the answer:

If the blue color results because short wavelengths are scattered preferentially, then why isn't the sky purple, since the wavelength of purple is even shorter?

cpt posted this earlier
cpt wrote:As an addition, to those of you who ask "Then why isn't the sky violet if the shortest wavelengths are scattered most?", it's because of our human eyes being much less sensitive to indigo/violet (purple) light than to blue.

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Re: colour of the sky

Postby thoughtfully » Fri Jun 03, 2011 4:52 am UTC

Color is really screwy stuff, but when you remember that it's all interpretation by our wetware, the Universe makes sense again (at the expense of biology). The Universe really exists as greyscales, not a combination of three not-even-orthogonal quantities. Thank the maker you're not a mantis shrimp!

ok, on second thought, it might be cool to have a few of those abilities!
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Re: colour of the sky

Postby Technical Ben » Fri Jun 03, 2011 7:54 am UTC

Just image school! "Red and yellow and ultraviolet and infrared... [and loads more colours you'll have to remember] ... orange and green". You'll have to name all the polarisations of them too. Then the teacher tells you you have a colour mixing and naming test in 5 mins. :shock:
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Re: colour of the sky

Postby evilspoons » Fri Jun 03, 2011 3:19 pm UTC

thoughtfully wrote:Thank the maker you're not a mantis shrimp!

ok, on second thought, it might be cool to have a few of those abilities!


OK, wow, that thing is freakin' awesome.

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Re: colour of the sky

Postby Technical Ben » Fri Jun 03, 2011 5:38 pm UTC

Don't let the cephalopods know you said that. They'll get envious and seek you out!
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Re: colour of the sky

Postby Essah » Mon Dec 10, 2012 7:33 am UTC

http://xkcd.com/1145/

my first Get out of my head Randall(tm) moment.

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Re: colour of the sky

Postby thoughtfully » Mon Dec 10, 2012 10:44 am UTC

My initial reaction is that there just isn't much violet in the solar spectrum. I suppose I should go look at the thread for that comic.
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Re: colour of the sky

Postby FancyHat » Mon Dec 10, 2012 12:41 pm UTC

thoughtfully wrote:My initial reaction is that there just isn't much violet in the solar spectrum.

Sounds reasonable. After all, it's not only violet that's scattered, but all other frequencies as well. Yes, they're scattered less, but they do occupy more of the spectrum, and I don't think violet light actually stimulates our red cones that much anyway (otherwise it would be more like magenta). So when you include all that green, yellow and cyan to get your green cones going, as well as all that extra blue that will get your blue cones going even more than your red cones, it's probably not much of a surprise that what would have been violet is lost in what turns out to be some shade of blue instead.
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Re: colour of the sky

Postby Essah » Tue Dec 11, 2012 9:10 am UTC

FancyHat wrote:
thoughtfully wrote:My initial reaction is that there just isn't much violet in the solar spectrum.

Sounds reasonable. After all, it's not only violet that's scattered, but all other frequencies as well. Yes, they're scattered less, but they do occupy more of the spectrum, and I don't think violet light actually stimulates our red cones that much anyway (otherwise it would be more like magenta). So when you include all that green, yellow and cyan to get your green cones going, as well as all that extra blue that will get your blue cones going even more than your red cones, it's probably not much of a surprise that what would have been violet is lost in what turns out to be some shade of blue instead.


According to discussion in the comic thread it is mainly due to limitations to the human eye making us percieve blue more easily than violet

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Re: colour of the sky

Postby FancyHat » Tue Dec 11, 2012 12:57 pm UTC

Essah wrote:According to discussion in the comic thread it is mainly due to limitations to the human eye making us percieve blue more easily than violet

Is this the post you mean? http://forums.xkcd.com/viewtopic.php?f=7&t=99191&start=80#p3223277.
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