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Tito_Muerte
2004-Mar-03, 03:58 AM
Is Raleigh scattering the red at sunset or the blue during the day?


And what's the name of the other type?

Ut
2004-Mar-03, 04:44 AM
According to my painfully dry quantum text book, both the red and blue are due to Rayleigh scattering.


The process that scatters photons without changing their wavelength is called Rayleigh scattering...

The other kind, I assume, is Compton scattering.

Tito_Muerte
2004-Mar-03, 04:51 PM
I could have sworn there were two names for it though, since the particles that split up the blue are a lot smaller (larger?) than the particles that split up the red.


What does your dry textbook define Compton scattering as?

Ut
2004-Mar-03, 04:56 PM
Scattering, via absorption and re-emission at a different wavelength. Pretty much the opposite as for Rayleigh scattering.

Here (http://en.wikipedia.org/wiki/Rayleigh_scattering), the Wikipedia has a rather enlightening entry about this.

Spaceman Spiff
2004-Mar-03, 06:05 PM
Is Raleigh scattering the red at sunset or the blue during the day?


And what's the name of the other type?

In regards to a point made above...
Rayleigh scattering is from bound electrons (atoms or molecules) and is conservative. Compton scattering is from (effectively) free electrons. It is not conservative, in general. Though if E_photon is much less than mc^2, where m is the electrons's mass, then the process is virtually conservative.

As I understand it....
In our atmosphere, there is insufficient airmass along the horizon to produce deep orange or red sunsets (sunrises), or for that matter ones that can be observed in comfort. A very clean molecular atmosphere (nearly absent aerosols, etc) would produce a yellowish/orange sun along the horizon, probably too bright to observe comfortably. Aerosol scattering (small particulates, sizes less than wavelengths of visible light, but much larger than single molecules), which often favors shorter wavelengths (roughly 1/lambda effect), plays a major role in the very colorful sunsets that are comfortable to observe.

swansont
2004-Mar-03, 06:16 PM
Mie scattering (http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html)

There is also inelastic scattering of photons, called Raman scattering.

George
2004-Mar-03, 06:23 PM
Is Raleigh scattering the red at sunset or the blue during the day?


And what's the name of the other type?

Yes.

The Rayleigh Scattering effect is the reason for the blue sky because blue light is scattered almost evenly in all directions. Therefore, the sky overhead looks blue as much more blue light is coming your direction.

The red sunset appears red because the blue light has been scattered away leaving the reds and oranges and yellows to come your direction.

There are other factors as well such as more dust at sunset and refraction.

The BA has a chapter on this in his book and you can check out more on this thread.... here (http://www.badastronomy.com/phpBB/viewtopic.php?t=8123). Also, in that thread as a bonus offer, is... S.A.D. (Sunlight Attenuating Device), the world's cheapest instrument ever invented. 8)

Rayleigh Scattering is an "elastic" process which means the wavelength of the light is unaltered. Other scattering effects such as Mie scattering are "inelastic". [Actualy, I don't really know what physically is happening in the elastic event, but someone around here probably does.]

It is interesting that Mars is just the opposite for apparently the same reasons. The larger particles scatter the red at sunset and the blue is what is seen around the sun.

Just to cloud-up the issue more, the color of the sun might actually be blue or is it "peachy-pink". (This is a personal goal of mine to learn).

Tito_Muerte
2004-Mar-03, 08:18 PM
MIE SCATTERING!!!


Thanks everyone, that's the word I was looking for. It's been on the top of my tongue for like...well, awhile.



Just to cloud-up the issue more, the color of the sun might actually be blue or is it "peachy-pink". (This is a personal goal of mine to learn).


Isn't that a factor of what wavelength our eyes are tuned to? And, seeing as how the sun emits most of its energy in the "visible" spectrum (which is as relative a term as I ever heard) wouldn't it....wait, now I've just confused myself.... #-o :-? :o

George
2004-Mar-03, 08:58 PM
MIE SCATTERING!!!

There are other scattering effects such as Raman (I think). I am curious of another which involves Einstein's efforts.




Just to cloud-up the issue more, the color of the sun might actually be blue or is it "peachy-pink". (This is a personal goal of mine to learn).

Isn't that a factor of what wavelength our eyes are tuned to? And, seeing as how the sun emits most of its energy in the "visible" spectrum (which is as relative a term as I ever heard) wouldn't it....wait, now I've just confused myself.... #-o :-? :o

The solar spectrum just outside our atmosphere has twice the irradiance in blue as compared to what we end up seeing down here due to the atmosphere effects (Rayleigh scattering only a small factor, surprisingly).
However, I believe that the Sun is too bright for us to "see" it's true color. Astronauts see it as blinding white. If all of our color cones are fully satrurated any object would appear white (so I think). Check out the link in the prior post for more. I'm kinda the resident trouble maker in this category although I am not "scientifically advantaged". :)

eburacum45
2004-Mar-04, 06:38 AM
There is Einstein-Smoluchowski scattering, or attenuation;

http://www.lns.cornell.edu/spr/2003-05/msg0051291.html

I don't know what it does, but I have heard of it a few times.

Something to do with fluctuating densities in the middle atmosphere.

Spaceman Spiff
2004-Mar-04, 03:16 PM
There is Einstein-Smoluchowski scattering, or attenuation;

http://www.lns.cornell.edu/spr/2003-05/msg0051291.html

I don't know what it does, but I have heard of it a few times.

Something to do with fluctuating densities in the middle atmosphere.

In essence, in regards to our lower atmosphere, it's Rayleigh (or rather, more precisely, bound electron) scattering!

Go to this paper (http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf)on atmospheric optics by atmospheric physicist, Craig Bohren (it's a pdf file, and adobe acrobat reader (http://www.adobe.com/products/acrobat/readstep2.html)is free). Many will find the whole thing really interesting; Craig writes in an engaging style. However turn to pages 57-58 (the paper begins with p.53) to see the discussion about Rayleigh scattering. He comments specifically on the "Einstein-Smoluchowski" paper on scattering due to large scale fluctuations in optically dense media, as introduced by eburacum45, above. For those of you who don't have the acrobat reader and have a slow modem, I'll copy the gist of it here:

from Craig Bohren, Atmospheric Optics (http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf), p. 58

To say that the sky is blue because of Rayleigh scattering, as is sometimes done, is to confuse an agent with a law. Moreover, as Young[3] pointed out, the term Rayleigh scattering has many meanings. Particles small compared with the wavelength scatter according to the same law as do molecules. Both can be said to be Rayleigh scatterers, but only molecules are necessary for the blue sky. Particles, even small ones, generally diminish the vividness of the blue sky.

Fluctuations are sometimes trumpeted as the ‘‘real’’ cause of the blue sky. Presumably, this stems from the fluctuation theory of light scattering by media in which the scatterers are separated by distances small compared with the wavelength. In this theory, which is associated with Einstein and Smoluchowski, matter is taken to be continuous but characterized by a refractive index that is a random function of position. Einstein [4] stated that ‘‘it is remarkable that our theory does not make direct use of the assumption of a discrete distribution of matter.’’ That is, he circumvented a difficulty but realized it could have been met head on, as Zimm [5] did years later.

The blue sky is really caused by scattering by molecules – to be more precise, scattering by bound electrons: free electrons do not scatter selectively.

For those interested in excellent discussions and explanations about what they see in the atmosphere, I highly recommend Craig Bohren's book, Clouds in a Glass of Beer (http://www.amazon.com/exec/obidos/tg/detail/-/0471624829/104-0309902-8775959?v=glance), though you should get this later edition from Dover (http://www.amazon.com/exec/obidos/ASIN/0486417387/ref=pd_ecc_rvi_1/104-0309902-8775959).

Let me just say that I am not plugging anyone. I've never met the guy, but his writings are excellent.

George
2004-Mar-04, 05:12 PM
Go to this paper (http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf)on atmospheric optics by atmospheric physicist, Craig Bohren...

Thanks. This looks excellent. I'll read it tonite.