# Why is the sky blue again?

#### 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

Staff Emeritus
2018 Award
Farsight said:
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..."

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#### ZapperZ

Staff Emeritus
2018 Award
Farsight said:
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

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#### ZapperZ

Staff Emeritus
2018 Award
Farsight said:
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

Gold Member
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

Gold Member
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

ZapperZ said:
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.

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#### ZapperZ

Staff Emeritus
2018 Award
So you have to actually cite someone who had to undergo a drastic change such as this to being able to view UV frequency? Why stop there? Why not have someone's eye be imbeded with night-vision sensor and then claim that human can also view IR spectrum?

What gives?

Zz.

#### Farsight

I thought that would be something suitably definite and credible, that was so obviously not a speculation that you would be willing to countenance that people have a range of capabilities, and the 400nm cutoff is not absolute.

#### Farsight

DaveC426913 said:
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.
OK, hands up, I could have phrased it better. How about:

The sky is blue because of something called scattering, wherein blue wavelengths are absorbed by molecules in the air then re-emitted in a different direction, whilst longer redder wavelengths tend to pass unobstructed. This isn't the same as reflection where something has a definitive colour, and it isn't the same as a rainbow where refracted light is split into a spectrum, but has a similar effect of separating the light into different colours. These vary with the angle away from the direction of the sun. When you can look directly at the sun (at dusk or dawn) you see more yellow and red, and when you look away from the sun you see blue.

#### ZapperZ

Staff Emeritus
2018 Award
Farsight said:
OK, hands up, I could have phrased it better. How about:

The sky is blue because of something called scattering, wherein blue wavelengths are absorbed by molecules in the air then re-emitted in a different direction, whilst longer redder wavelengths tend to pass unobstructed. This isn't the same as reflection where something has a definitive colour, and it isn't the same as a rainbow where refracted light is split into a spectrum, but has a similar effect of separating the light into different colours. These vary with the angle away from the direction of the sun. When you can look directly at the sun (at dusk or dawn) you see more yellow and red, and when you look away from the sun you see blue.
This is still not exact, because if you say that it is the "similar effect of separating the light into different colors", where are the other colors? Why only blue get the preferential treatment?

No, it is specifically Rayleigh scattering, and how our human eye has evolved, that are the significant factors. And this has been done rigorously, and not simply via wishy-washy, handwaving argument. See

1. B.R. Holstein, Am. J. Phys. v.67, p.422 (1999).
2. G.S. Smith, Am. J. Phys. v.73, p.590 (2005).

Introducing diffraction into this explanation simply adds to the confusion, especially when it has no role in the explanation.

Zz.

#### Farsight

If anbody could offer a brief Why the Sky is Blue explanation that I could relay to a child, I'd be grateful. I'd also be grateful if anybody could post any links as to why the sky isn't purple. I found this on google, but didn't follow the explanation:

http://www.livescience.com/forcesofnature/050719_blue_sky.html

Two spectra that have the same cone response are called metamers. Smith stressed that this only concerns the neural signal coming out of the eye - long before any processing by the brain. "In previous research, people excised cones from the eyes of dead people and measured the response to different spectra," he said. "The same trick that makes red and green turn into yellow is happening in the sky. But in this case, the sky's combination of violet and blue elicits the same cone response as pure blue plus white light, which is an equal mixture of all the colors..."

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#### Hurkyl

Staff Emeritus
Gold Member
ZapperZ said:
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 don't think it's even that good -- AFAIK there isn't any secondary effect from UV light here... it's just a plain, ordinary visible light phenomenon that happens to lie below the violet band.

#### Hurkyl

Staff Emeritus
Gold Member
The simplest description I've seen for why the sky appears blue is this:

The sky appears blue because air is blue.

All this talk about scattering light is analyzing too far -- the usual intent of the question is not asking for the underlying physical mechanisms that causes things to be colored, but instead expresses curiousity about the "fact" that air is colorless, and yet the sky is blue.

The easiest answer is that air, in fact, is not colorless, but is blue. But because the color is so faint, we only notice when we're looking through miles of air.

#### Cyrus

Hurkyl said:
The simplest description I've seen for why the sky appears blue is this:

The sky appears blue because air is blue.

All this talk about scattering light is analyzing too far -- the usual intent of the question is not asking for the underlying physical mechanisms that causes things to be colored, but instead expresses curiousity about the "fact" that air is colorless, and yet the sky is blue.

The easiest answer is that air, in fact, is not colorless, but is blue. But because the color is so faint, we only notice when we're looking through miles of air.

I am not so sure about that Hurkyl. If what you said were true, then as viewed from space, we would observe the earth to be primarily blue, and this is not the case. We can clearly and distinctly make out the Brown and Green terrain in contrast to the deep blue oceans. We do not observe a blue tint over land masses.

Further proof that scattering is the primary mode of why the sky is blue is due to the appearance of a red or orange sky at night. As the relative position of the sun changes, different wavelengths of light get scattered. This is why the sky is no longer blue, but red or orange.

...provided that I recall properly.

Over long distances, the air appears to wash out colors with a white or grey tint, not blue.

In the case of a Red sky, the angle of the sun is no long directly over head. The large angle results in the light having to travel through a much larger portion of the atmosphere. This causes the blue and violet spectrum to become lost, leaving the sky red, yellow, and purple.

As Zz already said, it is due to Rayleigh scattering.

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#### Doc Al

Mentor
Hurkyl said:
All this talk about scattering light is analyzing too far -- the usual intent of the question is not asking for the underlying physical mechanisms that causes things to be colored, but instead expresses curiousity about the "fact" that air is colorless, and yet the sky is blue.

The easiest answer is that air, in fact, is not colorless, but is blue. But because the color is so faint, we only notice when we're looking through miles of air.
I think you are mixing up air and water; "why the sky is blue" with "why the oceans are blue". The sky is blue primarily due to Rayleigh scattering (as mentioned repeatedly in this thread). On the other hand, I believe that water is blue, not colorless.

#### Farsight

You run into problems with blue sea and white clouds, hurkyl.

I believe it's the same effect for both air and water, Doc:

cyrus: I'm told you can emulate the situation at home with a bright light shining thought a tank of very diluted milk, wherein you see blue from the side, and orange looking through the tank up into the beam.

Anybody: If I overlap a blue light with a violet light do I see light blue?

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#### Doc Al

Mentor
Farsight said:
I believe it's the same effect for both air and water
I don't believe so. While air preferentially scatters blue light, water selectively absorbs red light.

#### Hurkyl

Staff Emeritus
Gold Member
I am not so sure about that Hurkyl. If what you said were true, then as viewed from space, we would observe the earth to be primarily blue, and this is not the case. We can clearly and distinctly make out the Brown and Green terrain in contrast to the deep blue oceans. We do not observe a blue tint over land masses.
I'm going by this explanation, which I particularly liked:

http://www.eskimo.com/~billb/miscon/miscon4.html#blu

What you say would only happen if the air was opaque, or had a very strong color.

Further proof that scattering is the primary mode of why the sky is blue is due to the appearance of a red or orange sky at night. As the relative position of the sun changes, different wavelengths of light get scattered. This is why the sky is no longer blue, but red or orange.
I never said that scattering is not the primary mode -- I'm saying that's simply analyzing the problem too deeply for the purpose of the question. Just like any other blue thing, air does something to light that causes us to see a blue color. Sure, the mechanism by which that happens is different for air, but I agree with the author of that page in that that's no reason not to simply say that air is blue.

#### Cyrus

I would like to point this out to you from his website:

ref-1 said:
They're wrong: you don't need complicated physics to understand this. The sky is blue for a very simple reason:

Air is not a perfectly transparent material. Instead it is blue!
Not quite. It may be true that air actually has some 'tint' to it; however, this is not the primary mode of why the sky is blue.

ref-2 said:
The sky is blue partly because air scatters short-wavelength light in preference to longer wavelengths. Where the sunlight is nearly tangent to the Earth's surface, the light's path through the atmosphere is so long that much of the blue and even yellow light is scattered out, leaving the sun rays and the clouds it illuminates red, at sunrise and sunset.

Scattering and absorption are major causes of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described, for spherical particles, by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.

Individual gas molecules are too small to scatter light effectively. However, in a gas, the molecules move more or less independently of each-other, unlike in liquids and solids where the density is determined the molecule's sizes. So the densities of gases, such as pure air, are subject to statistical fluctuations. Significant fluctuations are much more common on a small scale. It is mainly these density fluctuations on a small (tens of nanometers) scale that cause the sky to be blue.
I see what you are saying Hurkyl. The sky does have a component of blue that is inherent to it; but, I am saying that this is not the reason why the sky is blue and that his analogy of stacks of air making the sky blue is incorrect. If you extend his simple premise and continue it to its conclusion, how do you account for red sunsets? You simply can't.

Morever, he does not claim what you are saying, that scattering is the primary mode. He is saying the opposite, that scattering is NOT a mode.

That's flat out wrong.

ref-1:http://www.eskimo.com/~billb/miscon/miscon4.html#blu

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#### DaveC426913

Gold Member
Why the sky is blue - for the layperson. Submitted for your criticism:

The light from the Sun, though appearing white, actually contains many colours, including blue. Normally light travels straight. If the Sun's rays are not aimed directly at us, we would not see them at all. This is why space outside a spaceship is black, even when the Sun is shining.

So, because light rays normally travel straight, standing outside our house, the only light we *should* see would be coming *directly* from the sun, or bounced off nearby objects. The sky above us *should* be black because rays from the Sun *should* pass right over us, and so we wouldn't be able to see it at all. This is why the Moon's "sky" is black, even when the Sun is shining.

BUT, here on Earth, our atmosphere scatters some of the light, sending it in other directions than straight. It scatters reds the least, and blues the most. So, some of the light that *should* have gone right over our heads, missing us completely, actually is bent, and ends up hitting us. This happens more with blues than with other colours of the Sun's light.

When we look up at the sky, we are seeing rays of light (mostly the blue ones) that have been bent towards us by the scattering effect of air.

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#### russ_watters

Mentor
cyrusabdollahi said:
I am not so sure about that Hurkyl. If what you said were true, then as viewed from space, we would observe the earth to be primarily blue, and this is not the case. We can clearly and distinctly make out the Brown and Green terrain in contrast to the deep blue oceans. We do not observe a blue tint over land masses.
No, for the same reason why sunspots appear black: They are slightly cooler and dimmer than the rest of the sun, yet if the entire sun were the brightness of sunspots, it'd still blind you. They just get drowned out.

So the sky is bright enough against the backdrop of space to appear blue and not bright enough against the brightly lit earth to make it appear blue from space when looking directly at it.

Note, though, that on an extremely clear day, the sky looks much darker the closer to the zenith you look because you are looking through less atmosphere.

Note also that looking through much more atmosphere from space (like on the edges), the earth does take on a blueish tint, ie in South America (not to mention the atmosphere itself above Brazil) in this photo: http://visibleearth.nasa.gov/view_rec.php?id=2429 [Broken]

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