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Why does the beam end in the sky?

Izord

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I have a 100mw green laser and when I point it at the sky the beam 'stops' abruptly.

Whichever way I point it, unless it hits something like a tree or a powerline, but just pointing it in the empty sky it appears to 'stop'.

And always the same distance away. What causes this effect? It's as if there is a 'movie screen' or something up there stopping the beam.

I don't have a more powerful laser to see if it will project further up into the empty sky. Anyone with several different powers of laser checked to see if the more powerful one will seem to project a longer beam into the empty night sky?

Thanks. Izord
 



Things

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Could be heaps of things. Cloud layers, smoke, fog, temperature inversions or just plain limitation of the human eyesight.
 

Blord

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The laser light don't stop at any point. It will go on and on. You are looking at the horizon where all beams end. Just like railroads and highways. They "stop" at the horizon.
 

Things

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The laser light don't stop at any point. It will go on and on. You are looking at the horizon where all beams end. Just like railroads and highways. They "stop" at the horizon.

It will stop at some point, there is a lot of junk in the atmosphere it has to pass though.
 

Blord

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The laserlight actually reach the space station. An experiment was conducted with a positive results. Unless there is something in between like clouds or heavy pollution the light just shoots out into the space.

It has to do with the perspective of your vision. From your point of view it looks like the beam stops at a point.
 
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ElektroFreak

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In order to see a laser beam of ANY power, from .01mW to 100MW and beyond, there has to be particles in the air that reflect the light. If you were to take the most powerful visible laser in the world into a total vacuum and turn it on you would see no beam at all.

So, what's happening is that you are perceiving that the laser beam ends where the aerosol layer (the layer of air with the most particulate matter) ends. In fact the photons generated by the laser continue on to infinity if they are not blocked by something along the way.
 

Cyparagon

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It's more about field of view and trigonometry than it is about atmosphere. It would still appear to stop in an infinite atmosphere.

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ElektroFreak

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hmm.. could be, but then the visible length of the beam would (and does to a degree) depend on perspective I would think. If we shine a beam into the air, it appears from the ground to stop very suddenly rather than fading into oblivion, such as the way a long cable fades out of view. I can't put my finger on the reference, but the aerosol layer has been used as an explanation for this effect in publication. If I can ever find it I'll post the reference, not that it's accurate of course. To me the most sensible explanation would involve both the viewing angle and airborne matter.
 
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lasersbee

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The laser light don't stop at any point. It will go on and on. You are looking at the horizon where all beams end. Just like railroads and highways. They "stop" at the horizon.

Sorry... I will need to disagree with your assessment...
Railways and Highways seem to end at the horizon
because the Earth is curved and not flat..
A Laser beam is straight.

For one to see a laser's beam in the air (that has particulate
matter ie. dust.. humidity.. etc.) the beam must reflect off
those particles an reflect back to the operator.

The way I see it is that at one point the beam get weaker and
less light is reflected back yet the beam continues untill all
the photos are absorbed or reflected.

I may be wrong.. I've not done the experiment..


EDIT...

Looks like EF said the same thing...
I respond as I read down the Posts...:yabbem:


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
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Blord

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Sorry... I will need to disagree with your assessment...
Railways and Highways seem to end at the horizon
because the Earth is curved and not flat..
A Laser beam is straight.

That was a metafoor :)
If the earth was flat where do you think the rails end ? Right, still at the horizon. It converges to a point.

Look at this picture. All the buildings are standing straight but still they all point out to the same spot in the sky.
Imagine now the buildings are unlimited high. They stop middle in the sky.

SuperStock_1841R-97568.jpg
 

Wolfman29

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Interesting. Blord has a very good point. Never thought of it that way.
 

ElektroFreak

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That was a metafoor :)
If the earth was flat where do you think the rails end ? Right, still at the horizon. It converges to a point.

Look at this picture. All the buildings are standing straight but still they all point out to the same spot in the sky.
Imagine now the buildings are unlimited high. They stop middle in the sky.

SuperStock_1841R-97568.jpg

I don't know that they "stop" at the perceived center point as much as the space of open sky between them gets infinitesimally small to the point of not being perceptible from the ground.. they would seem to go on forever, with the patch of sky getting smaller and smaller but NEVER actually disappearing.
 
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Cyparagon

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I call BS on the "Planetary boundry layer." Beams don't look any longer when you point them toward the horizon.
 

benmwv

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I would agree that it's more about perspective. I have never seen a laser beam that seemed the end before the clouds, that sounds like total BS to me.
 

ElektroFreak

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Before we get to calling out any one theory as "BS" lets remember that the most sensible explanation would involve several variables, not just whether there is a boundary layer or just trigonometry etc, rather a combination of multiple factors. As with all science, simply going by what you detect with your senses is not enough.

Here's a good reference from a writer that I would consider qualified, pay close attention to section 2, entitled "Why does the beam from a green laser appear to "end" in the sky as compared to just going on forever?":

RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers

In particular this quote from Dr. Edward V. Browell, Head of the NASA Lidar Applications Group at the NASA Langley Research Center:

"We transmit high-power laser beams in the zenith on many occasions and observe the same optical effect you describe. The cause of this is the enhancement of aerosols (atmospheric particles) in the planetary boundary layer (PBL) which causes enhanced scattering of the laser beam back to your eyes. Above the PBL, which can be very low at night (<100 m), the amount of aerosols is very low compared to within the PBL, and as a result the scattering of the laser beam appears to end abruptly at the top of the PBL. More sensitive detectors, such as we use in Lidar, can continue to sense the scattering from aerosols and molecules well above the PBL demonstrating that the beam does not just stop there. If you are interested, you can see from our airborne lidar images that are posted on our web site (http://asd-www.larc.nasa.gov/lidar/lidar.html) how the aerosols (and scattering) vary across the atmosphere."
 
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