Why is the sky blue?

Cecil's Columns/Staff Reports
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It’s not scattered in all directions. If you’re orbiting earth - and you can check this out by looking at the international space station webcam the blue sky we’re used to seeing is completely invisible - form outer space looking in, there is no blue sky.

No, the blue is not shifted with more intensity than the other wavelengths. And it’s not really that abtruse. As photons from the sun arrive and collide with atmospheric gases, the gas atoms get a little momentum, and the photons lose a little of theirs - like ping pong balls. The angle of the path of the photon is deflected, as it’s lost energy it’s wavelength gets longer.

Because of atmospheric gas collision the entire spectrum shifted down - so invisible blues are shifted into the visible spectrum.

Since scattered light is the only reason the sky is illuminated at all (the airless lunar sky, as you no doubt recall, is black), what you see is blue.

Where is this blue diverted to?..What actually happens, is since the path of sun light through the atmosphere is longer, more photon/molecule collisions happen, the photons lose more energy and are shifted down, into red. (photons do not change speed like ping pong balls - light always travels the same speed - but their wavelength changes - the colour changes)

The sun is not normally yellow. From outer space the sun always appears white. Why we see a yellow sun on earth is because the atmospheric collisions shift the image of the sun down a little and it appears yellow. The red sun at night is when the image has been shifted down a lot more and it is now red (and there aren’t anymore invisible blues left to shift into the visible spectrum).

There is no lunar sky - something that is transparent by the grace of not being there is not black. But possibly one of the most important things to note from the scattering that causes the sky to be blue, and the sun redder at sun set, is that that energy which is missing from the spectrum has gone somewhere else. It’s the heat that makes the green house effect work. …People often get confused with these principles.

The bluish tint from glass is the same scattering effect - even the blueness of the sea.

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Since many people might not know what you are ranting about, giving a link to the article in question a damn good idea. Is it this one?

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You did your link wrong

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Okey-doke. Is that the one you are ranting about?

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I’m not ranting about it. Yes that is the link.

‘Why is the sky blue?’ is possibly the most common question a child will ask, but many of the answers are wrong or incomplete.

The correct answer is a very good demonstration of a number of important principles in physics.

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things can be scattered in all directions without being equally scattered in all directions.

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These statements are incorrect. While Doppler shifting does occur as a result of the scattering, it’s very slight, and you’ll get just as much blueshifting as redshifting on average. Cecil had it right: The blue is diverted more than the other colors. The blue that is missing from your sunset is making someone else’s sky blue.

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Yes, that is true. But in explaining something like the blueness of the sky, where the scattering angle is important (that’s what gives you the color, and why the sky does not appear blue from outer space).

The Tyndall effect (the blueness of milk and glass), Rayleigh and Compton scattering, they are all in principle the same thing.

The direction of the scatter is important. I think Cecil has a PhD in glass so he should know the Tyndall effect. But to use the term in all directions is wrong (to be pedantic, it is correct - but correct in the pedantic sense). If someone asks you why is the sky blue, if you respond that the light is scattered in all directions - and then you’re asked the question why is the sky white(gray) on a cloudy day - answering the light is scattered in all directions, is in fact more correct for a gray sky than a blue sky.

Scatter the light a little, you get blue, scatter it a lot and you get red, scatter it enough that you’re getting light scattered in all directions (and I mean that in terms of evenly in all directions) you will get white light.

But, his use of language is very incorrect for

I’m not really sure what a Texan can or cannot understand, but if you can’t explain it to a Texan - or an example a great scientist who can’t remember the name of; an Oklahoman corn farmer, then you probably don’t understand it yourself.

Saying the blue portion of sunlight is scattered with greater intensity than the other wavelengths, is, completely and utterly wrong.

The blue portion of light is scattered with less intensity than other wavelengths. This is the fundamental principle of the Tyndall effect - why glass appears bluish.

Of the visible spectrum, red photons are low energy end, blue the high energy end. The ‘blue sky effect’ demonstrates the wave particle duality of light (also that the speed of light is constant). To make more sense of what is happening, let’s just consider the photons as particles with momentum, the high momentum/energy photons are blue, the low momentum; red.

Consider the photon to be a billiard ball. And a gas particle to be another billiard ball. If you had a way of doing this little experiment in a very controlled manner, I would like you to roll the gas particle billiard ball at a constant speed, so it collides with the photon particle on it’s side - just as it’s passing. For the photon billiard ball, we can start at a low momentum - less energy. When the gas ball and the photon ball collide, the photon ball will be deflected from it’s original straight path. If you keep the momentum of the gas ball constant, but increase the energy you give to the photon ball, you will notice, that the higher the energy of the photon, the less it’s deflection.

The blue portion of sunlight is scattered with lesser intensity than other wavelengths. To be more precise, the bluer the light, the less it is scattered.

Another gem of this principle, ‘the higher the energy the lower the scatter’ is X-rays photography. They don’t have a magic property that allows them to pass through you any better than other radiation - but X-ray photons have such high energy, when they collide with something, like your flesh, blood and bone, they are scattered less intensely, there is less deflection in their path than lower energy photons - so you get a photographic image of your bones skull or whatever - when you place the photosensitive sheet at one side of the object, and the x-ray source at the other.

So ‘why is the sky blue’ is connected with ‘how do X-rays work’.

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Sorry, my statements are complete correct.

No, I wasn’t talking about Doppler shifting at all, at all.

The speed of light is constant. If a photon loses momentum in a collision, the particle velocity doesn’t change, but the photons frequency and wavelength to. It’s becomes shorter and its’ wavelength longer. Red is low frequency and blue high frequency. The frequency change in a collision is not the same as in a Doppler shift - the Doppler shift is a relativity effect.

No, it is diverted less - view my last post for a precise description and even a description of how x-ray photography works.

No, the blue is missing from the red sunset, because the photons have lost so much more energy crossing the earth’s atmosphere at that angle than when the sun is higher in the sky, and has less atmosphere to pass through. There are also red skies at morn for the same reason. The blue hasn’t become someone else blue sky. The wave function of a photon can only collapse in one place - so the light that comes from the sun to your eyes is just between you and sun and no one else.

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The only way the magnitude of a photon’s momentum can change in a collision is via the Doppler effect. And yes, the Doppler effect in light is relativistic, which is why it’s very very small for light traveling through the atmosphere: The velocities of air molecules are much less than the speed of light.

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No the Doppler effect has nothing to do with collision.

A photons energy can change in a collision. Photons do not have mass, but when considered as point particles they can be considered to have mass.

momentum = massXvelocity = mv

The integral of momentum is a particles kinetic energy. E = 1/2mv^2 , the energy of a photon is given as E = hf…(Planck’s constant multiplied by frequency)…But you can also give it as E = 1/2mv^2 , it doesn’t have mass but it does have momentum.

If a photon collides with a gas particle, which is what happens in a scattering collision, if the gas particle gains any momentum from the collision, the photon must lose some. Since it’s velocity is constant, it must lose some mass (it doesn’t really have mass to begin with - but it will have less momentum) Since Planck’s constant is a constant and cannot change the frequency of a photon must change after a collision. It has less energy so now must have a lower frequency.

No, I don’t really want to get into exactly how you’ve imagined this wrong, but the Doppler effect for light is related to the equation E = hf and that the speed of light is constant. And the equation E = p^2/2m (which is just a re-writing of E = 1/2mv^2 , with p standing for momentum). A relative motion effect would be you running at a baseball traveling at you - relative to you the balls is increasing in velocity, if you were running away it would be decreasing in velocity. Light cannot change its’ velocity - the speed of light is constant - but it can change its’ colour, more energy for blue, less energy for red. We get redshift with stars traveling away from us, blue shift for stars approaching us.

You can consider the colour of light to be its’ mass. If you don’t understand the equations I’ve used, you don’t fully understand the principle of the optical Doppler effect.

Air molecules cannot travel at the speed of light. But they can travel at speed, and have momentum. When they get zinged by light, their momentum increases.

Scattering collisions display the particle/wave duality of light - as far as the air molecule is concerned it has been hit by a particle and its velocity changes, its’ mass can’t - but the photons velocity can’t change, so it changes its’ mass - but it simultaneously wants to have zero mass and be a massless wave and changes it’s frequency instead of changing its’ mass. If you’re not confused, you don’t understand it.

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jmrcm: You’re going to have to provide some cites for your claims, because they are at odds with essentially every other source out there.

To a crude first approximation, EM radiation scatters most when the wavelength is the same as the the size of the object. Blue light scatters more than red because atoms are smaller than the wavelength of visible light, but blue is closer than red. High-energy X-rays scatter less because X-rays (mostly) have smaller wavelengths than the size of atoms.

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Dr. don’t pull the “peer-review papah please” stunt on me. It might make you look good when you’re arguing with a “climate change denier troll” - it’s a pretty reliable trick, as I can guarantee you both sides are generally clueless as to the underlying physics they’re arguing over. But it invariable makes the climate campaigner look good, as the the denier cannot call their bluff - because they invariably do not know what they’re on about - but neither does the smug climate campaigner. It’s trolls trolling trolls.
How about, Arthur Compton. He won the 1927 Nobel Prize for Physics, for the Compton effect (it’s named after him). His paper was peer-reviewed, and his peer’s even nominated him for a Nobel, which he won.

Your bluff has been called. There isn’t a single reputable modern physics book that will contradict me. Though the Internet is full of mangled tin hat wearing science, that isn’t science.

No. You’re mangling the principle of diffraction with the principle of scattering. Two very different principles.

Even though you confuse diffraction and scattering, you’re not even remaining consistent in your incorrect idea. Starting the spectrum from the lowest energy, you begin with the reds, and then as you move to a higher energy it’s blue, and then above that you’re into the x-rays.

The reds have the longest wavelengths, the blues it gets shorter, until you’re up into the x-rays where the wavelength is even shorter.

So, under your principle (whatever you want to call it - did anyone ever win a Nobel for it?), blue light scatters more because it has a shorter wavelength then red, but X-rays scatter less because they have a shorter wavelength than blue light. …Do you see where you have contradicted yourself, or would you like me…to…speak…slower…?

Dr. I’m sorry if I have hurt your feelings…But I believe you are the one who started the uncalled for rudeness…Never, ever, ever, ask someone to cite a “Peeror - revued - paypha”, if you do not know the peer-reviewed science that contradicts them.

In other words, never pretend you have a gun under your coat, because the other guy may really be carrying - and if you ask him to draw, he will and fill you full of smoking holes. And nothing wipes the smug look off a cowboy’s face quicker, or gives a cowboy more pain, than a belly full of hot lead.

I’m sure you’ve seen the westerns. A stranger walks into a saloon, and orders a sarsaparilla - a small town fool at the bar ridicules him - moments later the undertaker is arriving, the bar is full of gun smoke, and the stranger is enjoying their cool sarsaparilla in peace. I’ve been through a lot of small towns, pilgrim.

Do not be rude to me…… Do not argue science with me as a childish game…Unless you make a major scientific break through in the very near future, that contradicts a pretty rock solid body of established science, you don’t have an argument.
Now, all that said. ‘Why is the sky blue?’, once you have the correct answer is the key to understanding a large body of science - misunderstand it, and your mangled science will keep you locked out.

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Hi jmrcm, I think you misunderstand the culture here. This board has a large number of people working in or interested in the sciences who read or even publish papers as part of their daily grind.* A cite does not have to be a peer-reviewed paper, but if you’re unwilling to provide some sort of reference to back up your claims, this board might not be a great fit for you.

  • No, I don’t mean publishing daily :wink: I mean publication is an ordinary part of their work.
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jmrcm, asking for proof of your claims is not an insult and is not being rude. “Taking your word for it” is not the way science is done, no matter how learned you may be or think you may be. Get used to it, and provide cites up front, especially in the General Questions Forum. Even if you are right, a bombastic attitude without cites just makes you look foolish.

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jmrcm, the only one in this thread who seems rude to me is you.

Anyway, NASA says you’re wrong: Why Is the Sky Blue? | NASA Space Place – NASA Science for Kids

There’s my cite. Where’s your cite?

Also, if it appears blue because the light is losing momentum due to all the collisions with the air, why don’t stars appear blue? Why doesn’t the sun itself appear blue?

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Wait a minute – if the losing momentum story was correct, wouldn’t we see a red sky? Red light is lower energy than blue light, right?

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And now Marshall Dillon walks into the bar to see what all the fuss is about…:wink:

Moderator Note

jmcrm, personal remarks of this kind are not appropriate to this forum. Dial way back on the snark. You are free to make your arguments, but do it in a civil manner, and without all the gunplay. If someone is rude to you, report the post (triangle in upper right of the post) and let a moderator deal with it instead of taking matters into your own hands. No warning issued, but let’s refrain from this in the future.

[QUOTE=Musicat]
Since many people might not know what you are ranting about, giving a link to the article in question a damn good idea. Is it this one?
[/QUOTE]

Moderator Note

Musicat, this is inappropriately rude for this forum. Let’s give new posters the benefit of the doubt, OK? No warning issued, but don’t do this again.

Colibri
Comments Moderator

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In short, jmcrm, give us a reason to consider what you’re saying. If there’s a book that agrees with you, tell us what book, and quote from it. If you have a calculation that demonstrates your point, show us the calculation. If there’s an experiment that can demonstrate what you’re saying, describe the experiment. If you consider yourself personally to be an expert on this subject, then tell us your credentials (though this would carry little weight without an experiment or calculation, it’d at least be something). But if you just tell us “I’m right, and don’t argue with me, because I’m right”, we’re just going to ignore you at best, or laugh at you.

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Feynman Lectures on Physics, Volume I, section 32-5:
Earlier, we remarked that the phenomenon of scattering of light of this nature is the origin of the blue of the sky. The sunlight goes through the air, and when we look to one side of the sun–say 90 degrees to the beam–we see blue light…

The rest of the section goes on to describe the exact nature of the scattering. It has nothing to do with a frequency shift; only to do with the direction of the light changing.