<p>Rayleigh scattering is one kind of elastic scattering. Besides other kinds of <a href="http://en.wikipedia.org/wiki/Scattering" rel="nofollow">scattering</a>, <a href="http://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29" rel="nofollow">absorption</a> contributes to <a href="http://en.wikipedia.org/wiki/Extinction_%28astronomy%29" rel="nofollow">extinction</a>.</p>

<p>Absorption is dependent of the chemical composition of the dust or gas. But generally blue light is easier absorbed than red light, hence the reddening (not to be confused with red-shift!)</p>

CC BY-SA 3.0
2014-05-07T01:41:57.777Z
2014-05-07T01:41:57.777Z
Gerald
<p>In an astronomical context, Rayleigh scattering does not take a huge place.
The <a href="https://en.wikipedia.org/wiki/Extinction_%28astronomy%29" rel="nofollow noreferrer">extinction</a> is mostly due to absorption and scattering, from gas and dust. Its effect is to diminish the flux from the source (that is, a change in magnitude). Please, take care this depends on the observed wavelength!</p>

<p>For simplicity, we can assume the extinction factor $Q_{ext}$ as:</p>

<p>$Q_{ext} = Q_{scattering} + Q_{absorption}$</p>

<p>Where $Q_{scattering}$ is in turn composed of many elements.</p>

<p>As the OP above said, blue light generally suffers of higher extinction than other frequencies, so causing <em>reddened</em> observed light. This is due to the cross-section dependence on the frequency: if the light has a wavelength that is comparable to the size of the grain dusts (high frequency), it is more effectively absorbed, while in the opposite case it can pass through undisturbed.
This is the reason why obscured fields are observed in radio (in our galaxy, especially) or in infrared (extragalactic).</p>

<p>So, the answer to you question, <strong>in space</strong> is: Rayleigh scattering is not the most responsible of the extinction/reddening, you have to invoke absorption/scattering by dust and gas.
For comparison, <a href="http://www.astro.ncu.edu.tw/~wchen/Courses/ISM/11.Extinction.pdf" rel="nofollow noreferrer">here</a> some explanatory numbers about the <em>extinction efficiency</em> in different contests:</p>

<p>$Extinction \sim\lambda^{-1}$</p>

<p>$Thomson \sim\lambda^0$</p>

<p>$Rayleigh \sim\lambda^{-4}$</p>

<p>You basically would need impressive amount of gas to account for the extinction just by Rayleigh scattering.
Instead, <a href="https://en.wikipedia.org/wiki/Mie_theory" rel="nofollow noreferrer">Mie scattering</a> is more important in this context. Here a figure to quickly grab the concept (<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html" rel="nofollow noreferrer">from here</a>):</p>

<p><img src="https://i.sstatic.net/i29zG.gif" alt="enter image description here"></p>

<p>And here a very popular plot about extinction versus frequency <a href="http://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html" rel="nofollow noreferrer">from here</a>:</p>

<p><img src="https://i.sstatic.net/sqCzh.jpg" alt="enter image description here"></p>

<p>Of course, if you observe from ground, you should take care of Rayleigh scattering as well.</p>

<p>Other references:
<a href="http://astro.berkeley.edu/~ay216/08/NOTES/Lecture05-08.pdf" rel="nofollow noreferrer">Lecture 1</a>
<a href="http://astro.berkeley.edu/~ay216/06/NOTES/ay216_2006_07_Dust1.pdf" rel="nofollow noreferrer">Lecture 2</a></p>

CC BY-SA 3.0
2014-05-08T03:35:11.947Z
2014-05-09T00:36:46.297Z
2014-05-09T00:36:46.297Z
Py-ser
<p>I know from Rayleigh scattering that in the atmosphere redder photons are scattered less than bluer photons by the particles that both make up and are floating around in the atmosphere.  This is why the sky is blue and sunsets are red.  My question is, is the physics behind Rayleigh scattering (which, according to <a href="http://en.wikipedia.org/wiki/Rayleigh_scattering" rel="nofollow">wikipedia</a>, is due to the electric polarizability of the molecules) the same physics behind dust extinction? Additionally, is it the only physics in play or is there additional physics that causes the phenomenon?</p>

CC BY-SA 3.0
2014-05-07T01:35:44.093Z
Are you talking about the Earth's atmosphere, or interstellar/intergalactic extinction?
Py-ser
CC BY-SA 3.0
2014-05-07T16:53:27.840Z
I am talking about interstellar/intergalactic extinction.
NeutronStar
CC BY-SA 3.0
2014-05-08T00:22:14.283Z
Ok, I''ll try to arrange for an answer in a while
Py-ser
<dust>
Is dust extinction/reddening caused by Rayleigh scattering or some other physics phenomenon (or both)?
NeutronStar
# Is dust extinction/reddening caused by Rayleigh scattering or some other physics phenomenon (or both)?

(Asked by: username_0 on 2014-05-07)

I know from Rayleigh scattering that in the atmosphere redder photons are scattered less than bluer photons by the particles that both make up and are floating around in the atmosphere. This is why the sky is blue and sunsets are red. My question is, is the physics behind Rayleigh scattering (which, according to [wikipedia](http://en.wikipedia.org/wiki/Rayleigh_scattering), is due to the electric polarizability of the molecules) the same physics behind dust extinction? Additionally, is it the only physics in play or is there additional physics that causes the phenomenon?

**Tags:** dust
> **Comments:**
> **username_1**: Are you talking about the Earth's atmosphere, or interstellar/intergalactic extinction?  
> **username_0**: I am talking about interstellar/intergalactic extinction.  
> **username_1**: Ok, I''ll try to arrange for an answer in a while  

---

## 2 answers

### Answer by username_1 on 2014-05-08. Score: 5 | Accepted answer

In an astronomical context, Rayleigh scattering does not take a huge place.
The [extinction](https://en.wikipedia.org/wiki/Extinction_%28astronomy%29) is mostly due to absorption and scattering, from gas and dust. Its effect is to diminish the flux from the source (that is, a change in magnitude). Please, take care this depends on the observed wavelength!

For simplicity, we can assume the extinction factor $Q\_{ext}$ as:

$Q\_{ext} = Q\_{scattering} + Q\_{absorption}$

Where $Q\_{scattering}$ is in turn composed of many elements.

As the OP above said, blue light generally suffers of higher extinction than other frequencies, so causing *reddened* observed light. This is due to the cross-section dependence on the frequency: if the light has a wavelength that is comparable to the size of the grain dusts (high frequency), it is more effectively absorbed, while in the opposite case it can pass through undisturbed.
This is the reason why obscured fields are observed in radio (in our galaxy, especially) or in infrared (extragalactic).

So, the answer to you question, **in space** is: Rayleigh scattering is not the most responsible of the extinction/reddening, you have to invoke absorption/scattering by dust and gas.
For comparison, [here](http://www.astro.ncu.edu.tw/~wchen/Courses/ISM/11.Extinction.pdf) some explanatory numbers about the *extinction efficiency* in different contests:

$Extinction \sim\lambda^{-1}$

$Thomson \sim\lambda^0$

$Rayleigh \sim\lambda^{-4}$

You basically would need impressive amount of gas to account for the extinction just by Rayleigh scattering.
Instead, [Mie scattering](https://en.wikipedia.org/wiki/Mie_theory) is more important in this context. Here a figure to quickly grab the concept ([from here](http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html)):

![enter image description here](https://i.sstatic.net/i29zG.gif)

And here a very popular plot about extinction versus frequency [from here](http://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html):

![enter image description here](https://i.sstatic.net/sqCzh.jpg)

Of course, if you observe from ground, you should take care of Rayleigh scattering as well.

Other references:
[Lecture 1](http://astro.berkeley.edu/~ay216/08/NOTES/Lecture05-08.pdf)
[Lecture 2](http://astro.berkeley.edu/~ay216/06/NOTES/ay216_2006_07_Dust1.pdf)


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### Answer by username_2 on 2014-05-07. Score: 0

Rayleigh scattering is one kind of elastic scattering. Besides other kinds of [scattering](http://en.wikipedia.org/wiki/Scattering), [absorption](http://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29) contributes to [extinction](http://en.wikipedia.org/wiki/Extinction_%28astronomy%29).

Absorption is dependent of the chemical composition of the dust or gas. But generally blue light is easier absorbed than red light, hence the reddening (not to be confused with red-shift!)


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