Atmospheric Scattering

  • #1
I dont quite understand why we see sunsets as red/orange/yellow.

We see the sky as blue because Rayleigh scattering causes the smaller particles within the atmosphere to scatter bluer wavelengths and absorb others. But if the others are absorbed, which would include the red/orange/yellow wavelengths, how can we then see them in a sunset?

If during the day, the suns rays are scattered in the bluer wavengths and absorbed in the rest, doesnt that mean that the only visible rays that reach us only exist around the 450–475 nm range?

If then during a sunrise/sunset, which means the rays have more of our atmosphere to travel through (meaning more scattering and absorbtion), how do we then see the red/orange/yellow rays (570–750 nm)? Wouldnt those have already been absorbed?

Why doesnt our view of the sun also get affect by scattering? ie if the sun emits at least all visible wavelengths, which we see as white, When looking directly at the sun (well not directly of course), why dont we see the light without the blue wavelengths that have been scatterd? Which would make it less white.

Why does the http://www.freefoto.com/images/1223...=Big+Sky+Country,+Great+Plains,+Montana,+USA"look whiter towards the bottom?
 
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Answers and Replies

  • #2
Hootenanny
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We see the sky as blue because Rayleigh scattering causes the smaller particles within the atmosphere to scatter bluer wavelengths and absorb others. But if the others are absorbed, which would include the red/orange/yellow wavelengths, how can we then see them in a sunset?
See here: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html
If during the day, the suns rays are scattered in the bluer wavengths and absorbed in the rest, doesnt that mean that the only visible rays that reach us only exist around the 450–475 nm range?
No, if that was the case then we would only be able to see objects which reflect blue light and everything would look blue or black. Obviously we can see other colours. The light that is scattered by the atmosphere by large angles appears blue (making the sky appear blue), but that doesn't mean all the sunlight is scattered by large angles. Some is scattered by smaller angles and all angles in between, meaning we see the full spectrum. See here: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c1
 
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  • #3
Redbelly98
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We see the sky as blue because Rayleigh scattering causes the smaller particles within the atmosphere to scatter bluer wavelengths and absorb others.
Quick answer: the longer wavelengths are not absorbed, they are transmitted with very little scattering.
 
  • #4
Hootenanny
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Quick answer: the longer wavelengths are not absorbed, they are transmitted with very little scattering.
Far more succinct and informative that my post.

*Hangs head in shame* :frown:
 
  • #6
Hootenanny
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  • #7
I have. But I guess Im not grasping it.

scenario 1 :

During the day, the sun is up and rays, once entering the atmosphere go through a certain amount of scattering. The smaller particles will transmit rays mostly in the longer wavelengths and reflect mostly in the shorter wavelengths. What we see as the blue sky is the reflected wavelengths which are mostly made up of shorter wavelengths, hence the blue colour. The other longer wavelength paths aren't interfered with so much and continue their original path, by simply being transmitted throught the particles they intersect.

scenario 2 :

The sun is coming up, and rays once entering the atmosphere have a longer path, again, the rays are either reflected or transmitted through or off the particles they come into contact with. Because the rays have more atmoshere with which to travel through, more of these interactions with particles will occur. Which results in more of the shorter wavelengths being scattered out of the total rays, leaving the longer wavelengths which appear as red.

But.

The shorterlengths are scattered more, yes. But how does that cause less of them to be visbible leaving the longer wavelengths? Is there energy somehow lost with the greater number of scattering interactions which causes them to be less visible?
If longer wavelenths arent scattered as much, then shouldnt the red/orange colours we see in sunsets appear to spread much less across the sky?
 
  • #8
Redbelly98
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Your descriptions of scenarios 1 & 2 are accurate.
.
.
.
But.

The shorterlengths are scattered more, yes. But how does that cause less of them to be visbible leaving the longer wavelengths? Is there energy somehow lost with the greater number of scattering interactions which causes them to be less visible?
You're asking good questions. I have done a google image search, and found the following which may be helpful:


You see the sun because of the rays that travel directly from it to your eye in a straight line, without being scattered in some random direction. (Here I am ignoring the effects of refraction by the atmosphere.)

Blue light that comes from the sun directly toward you, but is scattered in a random direction, will not be received by your eyes. It is not lost, it simply goes somewhere else, other than to your eyes. So it will not appear, to you, to be part of the sun's image. Instead, it will be seen by somebody somewhere else, and will appear to them to come from "the blue sky" and not from the sun.

If longer wavelenths arent scattered as much, then shouldnt the red/orange colours we see in sunsets appear to spread much less across the sky?
I am trying to understand this question better. Are you refering to when there are clouds in the sky at sunset that appear red, or do you mean that a clear sky appears red at sunset?
 
  • #9
Thanks RedBelly, I thought you'd given up on me :)

I understand the whole concept of the shorter wavelengths being scattered around more, which causes blue light to end up being directed into our eyes, enableing us to see blue lighting from up around us when looking at the sky, and not directly at the sun.

I just dont understand the sunset bit.

In your image it makes complete sense, and so does the shining the light through milk experiment. In that if you're looking at the light source straight on, you see the light that has eventually reached your eyes after going through scattering, which mostly refects the shorter wavelengths in regard to rayleigh scattering. Which ultimatly means that the light that reaches you will have less shorter wavelengths than when it begun, giving a different percived colour, in this case red/orange/yellow. And that when you view it from an angle that is not straight on, you can see the light that has scattered, enableing you to still see light that eminated from the light source, despite not looking directly at it.

What I dont see is how we see large amounts of red/yellow/orange in the sky at sunset.

[PLAIN]http://www.lisisoft.com/imglisi/5/Screensavers/56804sunset.jpg[/CENTER] [Broken]

For that to happen then there must also be lots of scattering of the longer wavelengths too, else we would only see the red when looking at the suns volume, as is portrayed in the image you posted.

Basically, I see just as much scattering in the sunset image as the blue sky images. Yet Im told that longer wavelengths dont scatter as much.

And then finally, at sunset, we're told that the shorter wavelengths are scattered away, leaving the majourity to longer wavelengths. What does "Scatter Away" mean here?

The energy is absorbed or turned into some other energy that isnt light? ( conservation of energy )

If the shorterwavengths go through more scattering, I dont see how that makes them just go away.​
 
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  • #10
Redbelly98
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Hmmm, I've never thought about, or read an explanation of, why the sky can look red in the vicinity of a setting sun. Just taking an educated guess here, either there could be dust or other particles that get illuminated by the mostly red light hitting it -- or the blue has to travel through so much atmosphere at sunset, and is scattered so much, that much less of it reaches the observer.

If anybody knows for sure, feel free to chime in.
And then finally, at sunset, we're told that the shorter wavelengths are scattered away, leaving the majourity to longer wavelengths. What does "Scatter Away" mean here?

The energy is absorbed or turned into some other energy that isnt light? ( conservation of energy )

If the shorterwavengths go through more scattering, I dont see how that makes them just go away.
Scattering just means the light changes direction. It's not that it disappears, it simply travels in a different direction after encountering the molecule or particle that it scatters from. That is what is shown in the image in Post #8. And if that direction is down toward the ground, where it can be absorbed, or back out into space, then you do not see it.
 

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