Physics Q&A Game

Origin of blue sky?

 

[nbo10]

Rayleigh scattering

JMD

 

On what?

 

[marcus]

Originally posted by Alexander
On what?

That's really a second question. So I will put in a thought----it will still be nbo's turn because he said Rayleigh scattering.

I've always been assuming that the Rayleigh scattering that causes the blue of the sky was mostly from the outer electrons of the N2 molecule, just because nitrogen is the most common.

But it is not specific to nitrogen. Oxygen would work just as well and would look blue.

In fact if the Mars CO2 atmosphere were not so full of reddish dust it would look blue also.

So the question "off what?" seems as if it could be answered
"whatever light-element gases the air is made of"
or maybe one could just reply "the air".

 

[Tyger]

Off what?

Even air has an index of refractions, and it isn't homogenous. To my recall it's scattered off the irregularities in the atmosphere. It's also the cause of the Cyrillian blue we see when we look in the distance.

 

Originally posted by marcus
That's really a second question. So I will put in a thought----it will still be nbo's turn because he said Rayleigh scattering.

No, it is Tyger turn - he is correct.

Molecule of N2 is too small (d~0.3 nm) to give any essential contribution into blue sky color. Recall that intensity of scattered light is proportional to (d/[lamb])⁴, so it drops dramatically with size. It would take about hundred times more air to get essential contribution into scattering from individual molecules. But the distribution of molecules in air is fluctuating, and it turns out that most contribution into intensity of scattered light comes from fluctuating pockets of air of order of 10-100 nm in size. These pockets can have dramatically different number of molecules per unit volume.

Density of larger pockets does not fluctuates essentially, and there are less of those pockets, so despite their larger cross section their contribution is less.

Kinda interesting how interplay of math factors creates tangible phenomenon (blue sky) from something which is not very tangible itself (fluctuating due to thermal motion voids/concentration of certain size range).

 

Tyger, your turn.

 

[On Radioactive Waves]

Whats going on? This was too much fun to let the thread die. If Tyger doesn't want to go, someone else should pose a question.

 

[Ivan Seeking]

Originally posted by On Radioactive Waves
Whats going on? This was too much fun to let the thread die. If Tyger doesn't want to go, someone else should pose a question.

Is that your question?

Why don't you go.

 

[Tyger]

Sorry, I didn't know

that I got this one. I'll come back with a question when I think of a good one, hopefully today.

 

[Tyger]

Here's my question.

Why are metals stronger and more ductile than non-metals?

 

[nbo10]

electrostatic forces from the lattice of atoms that make of the metal

JMD

 

[marcus]

Originally posted by Tyger
Why are metals stronger and more ductile than non-metals?

this is not an answer exactly. I am thinking out loud: metals have comparatively small incomplete outer shells
and give up their outer electrons more easily than non-metals.

in a metallic lattice the outer electrons tend to be owned *collectively* by all the atoms---they are shared in common

such a lattice will be easier to deform (less rigid and brittle) and it will retain its integrity even while being deformed

in a non-metal lattice the ownership of the electrons is not widely shared but instead there is only sharing between nearby neighbors.
so deforming the lattice becomes a complex and difficult negotiation where specific bonds must be broken between old neighbors and reformed between new neighbors.

it is easy to see how cracks could develop and propagate in a non-metallic lattice

but "strong" is a word with various meanings and I suppose sometimes a carbon (non-metal) fiber could be strong---tho not ductile!

I yield to nbo, if there is any question about this, because he said "electrostatic forces" which covers all this in general tho it is not very specific---maybe I was just elaborating on nbo's earlier answer.

 

[Tyger]

I'm holding out

for a more specific answer. There is a more precise way of stating why.

 

[KillaMarcilla]

Man, I'm clueless

Although this page was a real interestin' read:
http://matse1.mse.uiuc.edu/~tw/metals/prin.html [Broken]

 

[Ivan Seeking]

Originally posted by Tyger
for a more specific answer. There is a more precise way of stating why.

Band Theory.
The energy separation between successive levels is extremely small - referring to the electron orbitals for each atom - due to sharing of unfilled orbitals.

 

[Tyger]

Now what to do?

The answer I was looking for was Surface Tension. The electron gas provides a powerful surface tension that holds the faces of the crystal latice together so that they slide over each other rather than break apart. We can see this surface tension at work in Mercury where the droplets resist deformation despite the high density.

 

[marcus]

Originally posted by Tyger
The answer I was looking for was Surface Tension. The electron gas provides a powerful surface tension that holds the faces of the crystal latice together so that they slide over each other rather than break apart. We can see this surface tension at work in Mercury where the droplets resist deformation despite the high density.

nice explanation. would not have occurred to me to think of
strength and ductility of metals due to surface tension of electron gas

unless there is some objection, why don't you ask the next question Tyger

 

[On Radioactive Waves]

I'll think of a question to post tommorow if someone dosn't beat me to it.

Once again, Tyger is lagging.

 

[marcus]

Originally posted by On Radioactive Waves
I'll think of a question to post tommorow if someone dosn't beat me to it...

that seems reasonable to me. I hope you do post a question.
The rules of the Q/A game are at the beginning of the "Astronomy" Q/A thread----I think "nicool" wrote them. The rules say something like it is up for grabs if the thread is cold for 3 days.
I forget exactly. But why stand on ceremony? Go for it.

 

[PeteGt]

It wouldn't just be raliegh, it's probably a little raman and raman-stokes. I would imagine the electrons would not fall back to their origonal energy levels everytime.

Pete

 

[Integral]

SD645,
The rules of the thread are to post a single question, the one who answers correctly (first) earns the right to to post the next question. At this point, while I am tempted to simply delete the extra questions I will instead specify that your first question is the current one on the "table". It would be nice if you cleaned up the other questions.

If you can't do it I can.

 

[sdeliver645]

Posting rules.

Sorry to have disobeyed the rules of the thread (single question posting and waiting my turn). Frankly, the thread isn't very active so I didn't think people would mind if I posted some interesting questions.

Guess I was wrong.

P.S. I saved you the trouble of "cleaning up" my questions.

 

[Integral]

Thanks,

Give it a bit more time. I am sure someone will step up to the plate soon.