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Why does the beam end in the sky?

It's my opinion along with that posed in the educated theorizing in the links that have been provided here that it's a combination of both factors, but the bigger contributor to this effect is the aerosol factor.
 
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ElektroFreak said:
If that is more convicing to you than a reply from someone educated in the field, and who works with this phenomena directly its obvious to me that this dicussion is pointless. I fully support free thought, but here a question was asked, verifiable answers from qualified sources provided by at least 2 people, but still people with comparatively little qualifications insist on other explanations. Sometimes people make my head hurt..
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ElektroFreak, I appreciate your answer as well. That is why I'm trying to formulate my experiment to determine the actual perceived length of the beam termination.

I already described the experiment, measuring the angle from reference point to beam termination point in the vertical direction, which should minimize the aerosol layer, and in a shallower angle to the surface of the earth which should maximize distance travelled through the aerosol layer.

I've observed casually that the distance seems to be the same in either orientation, so I'll obtain some numbers. ie hard science. And I think moving the measurement point some 100 meters from the laser should help to increase the gain over perspective effects.

Hope that makes your head hurt less. :yh:
 
Actually the answer I provided was not mine, I just provided a reference to an educated position on the topic. Having done research into this I've seen the data I provided before. IMO the idea that this phenomena is caused by aerosols with a little influence from viewing angle is the simplest explanation. Most of the time the simplest explanation that explains all or the majority of uncertainties is the best one, the closest to correct. There are exceptions of course, but it's a good rule of thumb.

Cyparagon's edited data supports the data provided in the previous links also unless I'm misunderstanding. He attempts to prove (convincingly I might add) that from the vantage point of the ground near the laser source it is nearly impossible to tell with certainty if the visible portion of the beam becomes longer as it's angle moves from vertical toward the horizon. Therefore it could be (and likely is based on the explanations in the links) getting significantly longer.
 
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edit: oops, better read recent posts before posting...
 
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ElektroFreak said:
Actually the answer I provided was not mine, I just provided a reference to an educated position on the topic. Having done research into this I've seen the data I provided before. IMO the idea that this phenomena is caused by aerosols with a little influence from viewing angle is the simplest explanation. Most of the time the simplest explanation that explains all or the majority of uncertainties is the best one, the closest to correct. There are exceptions of course, but it's a good rule of thumb.

Cyparagon's edited data supports the data provided in the previous links also unless I'm misunderstanding. He attempts to prove (convincingly I might add) that from the vantage point of the ground near the laser source it is nearly impossible to tell with certainty if the visible portion of the beam becomes longer as it's angle moves from vertical toward the horizon. Therefore it could be (and likely is based on the explanations in the links) getting significantly longer.
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I guess it's time for some measurements. Wish I had my pop's Brunton tonight.

Next week I'll have another laser to compare, but has anyone with two or more different powered lasers compared beam lengths? Maybe do it tonight?

Thanks, Izord.
 
I'm a little hazy as to how you'll find the exact point (or close enough to it) at which the beam's visibility is reduced.. triangulation had occurred to me, but that would be quite a challenge I would think given the fact that the boundary layer's edges aren't well defined. It looks sudden to us from our perspective, as if the beam just stops, but in reality it's likely a fairly gradual transition. In order to take a measurement from a distance that would allow for some accuracy, the laser would have to be extremely powerful. A laser beam is difficult to see from an angle perpendicular to it, and in the case of vertical or near-vertical beams the further you move out from the source at ground level the closer you get to perpendicular to the beam. The further out you go, the more power you'll need to be able to clearly see the beam. Realistically we'd be talking about 30-50W of green before you'd be able to clearly see the beam from any angle useful to this experiment, and that's most likely a very conservative guesstimate.
 
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I saw no mention of viewing angle in the links (correct me and point me to it if it's there). I claim the biggest reason the beam stops is viewing angle and that atmosphere only comes into play when you are a remote observer, and even then, only to a small extent.

I'll use this picture to further explain:

boundary-layer.jpg


Note the scale. You would be less than a thousandth of a pixel from the base of the beam.

Here's one way you could test it, based on the picture. Zoom in on the area just after the beam stops and take a long exposure. EF claims you'll see a beam, I claim you'll see none.
 
Cyparagon said:
I saw no mention of viewing angle in the links (correct me and point me to it if it's there)
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Section 3, entitled "The Atmosphere and Astronomy", covers viewing angles and their effects as they pertain to astronomy, so the article doesn't apply the math to this specific phenomenon directly but when I consider the math presented and I see the same logic we're discussing here used to quantify atmospheric thickness relative to the observer, to me there are obvious implications. RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers
 
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Cyparagon said:
Note the scale. You would be less than a thousandth of a pixel from the base of the beam.
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The picture you gave had no scale. And the link EF posted stated that the PBL could be as low as 100m. In no way is this "1/1000 of a pixel"
 
nouthyella said:
In no way is this "1/1000 of a pixel"
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That was an off-the-cuff guess, but I'll go through it if you like. As little as 100m, yes, but more realistically 450m considering the urban location, according to wikipedia. There are 50 pixels between the ground, and the dotted line. If 450m corresponds to 50 pixels, that means each pixel represents 9 meters.

2 feet would therefore be 1/15th of a pixel. I stand corrected. :shhh:
 
FYI just tested with 3 lasers:
445nm 1.1W
532nm "300mW" (more like 100)
638nm 415mW

They all end together as expected. I'd take a picture but 3 lasers fill my hands :P
 
I have read this interesting discussion
And feel all factors must come into
Consideration when pointing a laser
At the sky.
1. Atmospheric Conditions
2. Particle Density in the Aerial heights
3. Angle of laser beam relative to Eye
4. Strength Wattage Nm and Color of Laser
6. Particle Absorbtion
7. Perspective Of Line Of Laser
8. Distance the Eye can 'See'

We see the stars but the light
is travelling toward us
Not away from us as our lasers are
The further and weaker the stars are
The less we see them
Essentially even their light 'stops'
A laser is only often millimetres across
How far we see the line of the laser light
That is a only a few millimetres across
Depends, I think, to my unscientific find
To all the above factors ...
And also in all my
amatuer scientific study I have learned this
One important lesson, and that is not
To take anything NASA tell you as
Definite Scientific Proof.
Let us deduce the correct theorem
And proofs for ourselves and make our
Own conclusions and not just cut and
Paste some so called learned knowledge
as blind cemented fact.Astronomers
only in the 1870's thought the moon
was 37 miles away and 100 meters wide.
please use your lasers of different strength
And color
And make measurements and see if this
Phenomenon can be solved, concluded
As I think there may be other factors
That we have overseen.
 
I agree with that last part believe it or not.. but the fact is that the word of a guy at NASA is the best actual reference that's been presented here so far. If you want to do research well, then you have to pay attention to your sources. This specific topic is pretty hard to find a lot of concrete data on in my experience. It's like I said before: Scientific explanations, hypotheses and theories are not always 100% accurate. All that we have as curious minds seeking the real answers to our questions is the ability to judge our sources for ourselves based on our own gut, which in turn is often rooted in our own knowledge and experience.
 
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