Why Does the Sky Appear Blue During the Day?

[vincikai]

okay, i forgot why is the sky blue again?

 

[Danger]

Scattering. The refractive index of the atmosphere tends to spread the bluish part of the spectrum around, while blocking other parts of it. That's a pretty sad explanation, but I hope that it will hold you until the big guns come around. Welcome to PF!

 

[gnomedt]

Well, this isn't the best explanation either, but HowStuffWorks has some reasonable links:

http://science.howstuffworks.com/question39.htm

 

[Farsight]

The sky is blue because we kind of live at the bottom of a rainbow, only it's an "atmosphere bow".

Look at the colours on a rainbow. Blue is at the bottom. That's been refracted most. You can see the other colours that get refracted less at dusk or dawn. Sometimes you can even see what's called "the green flash".

http://mintaka.sdsu.edu/GF/

 

[Gokul43201]

okay, i forgot why is the sky blue again?

What is your current educational level?

 

[dav2008]

okay, i forgot why is the sky blue again?

It's blue again because the sun rose this morning.

 

[DaveC426913]

The sky is blue because we kind of live at the bottom of a rainbow, only it's an "atmosphere bow".

I love this logic!:rofl:

So, how high would you have to climb for the sky to be green or yellow?
If we dug a hole, and looked up from the bottom, would we see an Indigo or Violet sky?

 

[Farsight]

Dave, it's a nice simple analogy. Hits the spot, you know?

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

"If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works..."

 

[disregardthat]

dave got a pretty good point i think. Isnt the sky blue because the air molecules reflects mostly blue light? that will say: lightbeams with the wavelength that we see as the color "blue"

 

[Doc Al]

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

That's a good link--read it. But the blue sky has nothing to do with us "living at the bottom of a rainbow", whatever that might mean.

 

[Farsight]

Have a look at this picture of a rainbow, Doc. The blue is the bottom stripe. The red and yellows of dawn/dusk are at the top. And there isn't much green.

http://www.sundog.clara.co.uk/rainbows/images1/shim18.gif

 

[Farsight]

I seem to having problems with images. Please search google, or get the hose out on a sunny day. Here's one where the violet shows better than normal. Interestingly you can also see the ultra violet. Normally you have to use the side of your eye for this, whereupon you are aware of a rather transparent yellow glow.

http://photos1.blogger.com/blogger/...80506200550.photo03.photo.default-380x512.jpg

 

[Hootenanny]

Interestingly you can also see the ultra violet.

Wait, you can see UV light?

 

[Farsight]

Yep. Look at the photo. There's a half-width band of pale translucent yellow under the indigo. With a real rainbow you have to turn your head sideways before you can see it. It isn't obvious, it's rather like an "after image" colour when you close your eyes after looking at a bright light.

http://photos1.blogger.com/blogger/...80506200550.photo03.photo.default-380x512.jpg

 

[Hootenanny]

Yep. Look at the photo above.

Nope, you cannot see ultra violet light. However, you have see visible violet light - $\lambda ~ 400nm$.

 

[Hurkyl]

Yep. Look at the photo. There's a half-width band of pale translucent yellow under the indigo. With a real rainbow you have to turn your head sideways before you can see it.

I zoomed in on the image as close as I can with my browser, and I can't find any yellow pixels below the violet band.

Of course, I didn't expect to see any UV light anyways, since my monitor displays colors with a mixture of red, green, and blue. :tongue:

It isn't obvious, it's rather like an "after image" colour when you close your eyes after looking at a bright light.

Maybe it's more than merely "like" that. :tongue:

 

[Farsight]

Look at the photo Hootenanny. And take a sidelong look at a rainbow next time you see one. It ain't violet.

 

[Hootenanny]

Look at the photo Hootenanny. And take a sidelong look at a rainbow next time you see one. It ain't violet.

I don't disagree that the photo shows violet light. However, I disagree with the fact that you said it was UV light, UV light is outside the visible spectrum, therefore we cannot see it. We can however, see visible violet light. In addition, I have seen a rainbow with a purple stripe before.

 

[Farsight]

You can't actually see a colour. All you can really see is a brightness inside the violet, but you have to use the side of your eye. There is definitely some light in this region where the UV ought to be. Search google on "interference bows".

 

[Hurkyl]

Interference bows aren't ultraviolet light.

 

[dav2008]

I don't disagree that the photo shows violet light. However, I disagree with the fact that you said it was UV light, UV light is outside the visible spectrum, therefore we cannot see it. We can however, see visible violet light. In addition, I have seen a rainbow with a purple stripe before.

Exactly. By definition you can't see UV light.

 

[Farsight]

Interference bows aren't ultraviolet light.

I know, hurkyl. But have a look again for some yellow pixels. You will find them. That's step one.

By definition you can't see UV light.

Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.

 

[Hootenanny]

Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.

Go to google and type in 'define: UltraViolet light'.

Ultraviolet light has a wavelength of $100nm < \lambda < 400nm$, the shortest visbible wavelength of light our eyes can percieve is 400nm therefore, I repeat again UV light is not visible to the naked eye under any cicumstances

 

[ZapperZ]

Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.

Under what "abnormal" condition are you proposing that YOU can "see" UV light?

Zz.

 

[Hurkyl]

I know, hurkyl. But have a look again for some yellow pixels. You will find them. That's step one.

If you know that interference bows are not ultraviolet light, then why the heck did you bring them up?

Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.

You might want to check the definition again. e.g. the first three Google definitions, or the first sentence at Wikipedia.

 

[Farsight]

Zapper: you get your hosepipe out in the bright sunshine, and turn the nozzle to a fairly fine mist, preferably in front of a dark area like an open garage door. You position the spray and yourself so you can see the rainbow you're making. Now you turn your head sideways to look at the rainbow with your peripheral vision. You should be aware of a brightness below the violet band.

All: we all know bees can see UV. And diurnal birds and rats and bats have some degree of UV perception.

http://www.mpih-frankfurt.mpg.de/global/Na/eindex.htm

 

[ZapperZ]

Zapper: you get your hosepipe out in the bright sunshine, and turn the nozzle to a fairly fine mist, preferably in front of a dark area like an open garage door. You position the spray and yourself so you can see the rainbow you're making. Now you turn your head sideways to look at the rainbow with your peripheral vision. You should be aware of a brightness below the violet band.

Wait... and you are saying that this IS the UV spectrum?

I work with a UV laser, 248 nm wavelength to be exact. It is perfectly invisible to me, which makes it even more dangerous, especially since it's a Class IV laser. How do I detect it when I do laser allignment? Using a business card! The coating causes luminescence on the surface of a business card and this makes a visible light that *I* can see. However, this does not mean I can actually see UV light. All it means is that the interaction of UV light with something causes light in the visible range to be produced that my eyes can pick up.

Do you see the difference here?

All: we all know bees can see UV. And diurnal birds and rats and bats have some degree of UV perception.

http://www.mpih-frankfurt.mpg.de/global/Na/eindex.htm

Last time I checked, the "visible spectrum" was defined by human optical range, the same way "audible range" was defined by human hearing range. Unless you are a bat, you have brought up something completely irrelevant here.

Zz.

 

[Farsight]

I'm not sure of the difference between UV vision and UV perception or whether florescence is involved. And I can't vouch for this, but I think it's interesting:

http://www.sciencedaily.com/releases/2003/10/031017073642.htm

"Bats from Central and South American that live on the nectar from flowers can see ultraviolet light (Nature, 9. October 2003 p. 612-614). This was discovered by York Winter, a German researcher at Munich University and the Max-Planck-Research Centre for Ornithology together with colleagues from Germany and the University of Guatemala. As bats generally lack cone pigments in their eyes, the flower bats capture the ultraviolet with the rhodopsin of their rod pigments..."

 

[Farsight]

Zapper: 248nm does sound very short. Have a look at "shortest visible wavelength" and note the "except for a few".

http://www.rattlesnake.com/notions/birds-color-vision.html

 

[ZapperZ]

Zapper: 248nm does sound very short. Have a look at "shortest visible wavelength" and note the "except for a few".

http://www.rattlesnake.com/notions/birds-color-vision.html

Why is this relevant? All I care about is that I'm in the "UV" range, and last time I checked, 248 nm is right fat smack in this range.

Zz.

 

[Farsight]

It's relevant Zapper, because this is a Physics Forum where we all enjoy , debate, thought, and learning. And statements like:

Ultraviolet light has a wavelength of $100nm < \lambda < 400nm$, the shortest visible wavelength of light our eyes can percieve is 400nm therefore, I repeat again UV light is not visible to the naked eye under any cicumstances

...deserve a response that says there are variations in human visual perception, there is no magical cut-off at precisely 400nm, some people can see further into the ultraviolet than others.

http://starklab.slu.edu/humanUV.htm

 

[ZapperZ]

It's relevant Zapper, because this is a Physics Forum where we all enjoy , debate, thought, and learning. And statements like:

Ultraviolet light has a wavelength of $100nm < \lambda < 400nm$, the shortest visible wavelength of light our eyes can percieve is 400nm therefore, I repeat again UV light is not visible to the naked eye under any cicumstances

...deserve a response that says there are variations in human visual perception, there is no magical cut-off at precisely 400nm, some people can see further into the ultraviolet than others.

But you're making a speculation here because (i) you haven't shown someone who can actually see in that range and (ii) you were using invalid examples to somehow support your argument that yes, we can see the UV range. That is what I was trying to counter. Your examples are not valid, simply because what you think as a direct observation of a "UV" light is really a secondary effect the same way that I am "viewing" my laser using business cards.

I would love to find someone who can view UV range. It will make my life so much easier when I have to do reallignment, which is very often.

Zz.

 

[DaveC426913]

And getting back to the original thread topic, saying "we live at the bottom of a rainbow" is an erroneous analogy.

Other than pointing out that the blue in the sky is related to the bending of light in a rainbow, it does nothing to illuminate the user's understanding of the phenomemon. In fact, it steers them in an erroneous direction (as in my example) if they attempt to uindestand the analogy.

 

[DaveC426913]

Farsight, has it occurred to you that there could be other explanations for the phenomenon of a bright band of light past the violet band? Has it occurred to you that the refraction of light in water droplets might not be a perfect continuum, and might behave differently near the edges?

 

[Farsight]

But you're making a speculation here because (i) you haven't shown someone who can actually see in that range and (ii) you were using invalid examples to somehow support your argument that yes, we can see the UV range.

People have a range of capabilities which invalidates a hard-and fast cutoff at 400nm. If you want to find somebody who can definitely see into the UV range just look at aphakia:

http://money.cnn.com/magazines/fortune/fortune_archive/2005/04/04/8255929/index.htm

WHEN WILLIAM STARK WAS 10 years old, a nail he was hammering in the cellar of his home in Pittsburgh shot upward and plunged directly into his left eye. His injury was bad enough that a surgeon eventually had to remove the lens. In an intact human eye, the lens filters out ultraviolet light, which can injure the eye over time. So when Stark's lens came out, UV light for the first time could make it all the way to his left retina. "I suddenly was seeing a whole bunch of the spectrum that I hadn't been able to see before," he recalls. A little like Superman, little William Stark had UV vision. Ever since then, Stark's world has been painted according to a different plan. It's generally brighter and bluer. The sun appears more brilliant, the moon richer in off-whites.