Why Is the Sky Blue?

[merrit lim]

why sky is blue in colour??

why sky is blue in colour??

 

[Ken G]

The reason the sky emits any light at all is because it is scattering sunlight. The particles responsible for scattering sunlight typically are best at resonating with light at a much higher frequency than sunlight, so they see the electromagnetic waves coming from the Sun as rather "slow." In a classical description, this means the restoring forces on the electrons responsible for scattering that light is easily able to stay in close balance with the electric forces coming from the wave. That in turn means that, for a given intensity of light of either red or blue color, the amplitude of the oscillation of the electron is the same for both colors (think of a little springs that deliver forces in proportion to the distance they are stretched, so same force, same distance, same amplitude of oscillation).

Now you might think if red and blue light were creating the same amplitude of electron oscillation, they would scatter equally, but that's not the case-- the amount of light emitted by the electron depends on the square of its acceleration, so if you have oscillation at some frequency f, the square of the acceleration scales with f⁴. That means light at a factor of 2 higher frequency (like very blue light compared to very red light) will actually be emitted 16 times more. This is called "Rayleigh scattering."

 

[DrStupid]

why sky is blue in colour??

http://en.wikipedia.org/wiki/Rayleigh_scattering

 

[Ken G]

To me, that Wiki entry is somewhat wanting. For example, early on, it says "It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases", yet the only equations it gives relate to particles with a refractive index. It seems to ignore how atoms and small molecules (like nitrogen) Rayleigh scatter. Also, it claims to "explain" how the process works, but in fact it only refers to a formula, and claims that the formula is an explanation. I appreciate that the article is kept short and sweet, but to me, it really doesn't explain much of anything about why the sky is blue.

 

[DrStupid]

It seems to ignore how atoms and small molecules (like nitrogen) Rayleigh scatter.

Rayleigh scatter is not limited to atoms and molecules. It can be caused by any small structure e.g. crystallographic defects. Therefore an detailed explanation should not be limited to molecules.

 

[Ken G]

That is not what I said-- I pointed out that the Wiki is limited to particles with an index of refraction. It does not treat how atoms and small molecules (like nitrogen) Rayleigh scatter, which is rather important for the blue sky in clear conditions. More importantly, by focusing on a derived formula rather than the fundamental physics, it fails to unify the key element of all Rayleigh scattering-- scattering due to charge oscillations with amplitude that has no intrinsic dependence on frequency.

 

[yenchin]

There's actually a pop-science book on this: https://www.amazon.com/dp/0262162342/?tag=pfamazon01-20.

 

[Andy Resnick]

why sky is blue in colour??

Is it? Is the sky always a single shade of blue? When (or where) is it not blue?

Answering these questions will help you understand the mechanisms.

 

[klimatos]

Now you might think if red and blue light were creating the same amplitude of electron oscillation, they would scatter equally, but that's not the case-- the amount of light emitted by the electron depends on the square of its acceleration, so if you have oscillation at some frequency f, the square of the acceleration scales with f⁴. That means light at a factor of 2 higher frequency (like very blue light compared to very red light) will actually be emitted 16 times more. This is called "Rayleigh scattering."

Woops! I have always believed (and taught) that the blue color of clear daytime skies was the result of scattering of photons having "blue" wavelengths. You seem to imply that it is also the result of emission of blue light by atmospheric constituents. If I am interpreting what you said correctly, could you please provide a citation.

 

[Ken G]

The emission I refer to is just the tail end of the scattering process that you refer to. In a classical description, the incident light causes the electrons in the particles to oscillate, and that in turn causes emission, according to the Larmor emission formula. The key point is that if the frequencies involved are "slow" from the perspective of the atom's natural response period (or the particle's light crossing time), then the oscillation is essentially in force balance, and so its amplitude has no intrinsic dependence on frequency, it only depends on the field strength. Then the scattering of "white" incident light will depend only on the fourth power of frequency, appropriate to the Larmor emission formula whenever the amplitude is fixed.

 

[Drakkith]

What's it called when you explain something in terms so advanced the person probably has no chance in understanding? I think that might have happened here.

 

[klimatos]

Ken G,

I "think" I understand most of what you are saying, although the needle on my "jargon meter" is pinned. However, I am still waiting for a citation.

 

[Ken G]

I don't understand, why would I cite something in support of something I did not say and is not true? What you taught your students is correct-- the blue sky is scattered light, just as I said above. What I said would be confirmed by any basic physics textbook that endeavors to actually explain the root cause of the microphysics of Rayleigh scattering, and not just cite a formula involving indices of refraction (which are derived from the microphysics anyway). Also, the only jargon I invoked is the Larmor formula, which seems pretty essential to the issue.

 

[Drakkith]

Ken G,

I "think" I understand most of what you are saying, although the needle on my "jargon meter" is pinned. However, I am still waiting for a citation.

He is just describing part of how scattering works I believe. It's still scattering, not emission.

 

[klimatos]

I don't understand, why would I cite something in support of something I did not say and is not true? What you taught your students is correct-- the blue sky is scattered light, just as I said above. What I said would be confirmed by any basic physics textbook that endeavors to actually explain the root cause of the microphysics of Rayleigh scattering, and not just cite a formula involving indices of refraction (which are derived from the microphysics anyway). Also, the only jargon I invoked is the Larmor formula, which seems pretty essential to the issue.

Ken,

I surrender. I freely admit that you obviously know more about electron physics than I do.

In my weak defense, I can only say that I was thrown by your use of the terms "emitting" and "emitted" in your post #2. I had always considered molecular emissions to be quite different from molecular scattering. Obviously I was wrong.

 

[Ken G]

The term "emission" does get used in different ways, so you are right to want to make it very clear exactly how that term is being used. "Absorb" is even worse-- some people think of scattering as an "absorption-and-emission" process, but others reserve "absorb" for when the photon is destroyed and any new emission is a new process.

 

[klimatos]

The term "emission" does get used in different ways, so you are right to want to make it very clear exactly how that term is being used. "Absorb" is even worse-- some people think of scattering as an "absorption-and-emission" process, but others reserve "absorb" for when the photon is destroyed and any new emission is a new process.

As far as "absorption" goes, the latter has always been my understanding. I understood that absorption caused the photon to cease to exist. Perhaps it's simplistic, but I used to teach that when the interaction of a photon with an air molecule caused the path of the photon to change, it could be considered as "scattering". When the photon's path remained unchanged and the photon continued to exist after interacting with an air molecule, the "transmission" was said to occur.

For the purposes of developing atmospheric heat budgets, this level of sophistication appeared to be adequate.

 

[Ken G]

Yes, those are very common usages of those terms.

 

[davidscott5]

The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere


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