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# Why does the beam end in the sky?

#### n2stuff

##### New member
That is EPIC..
Yup. I read it on a photo copy many years ago. I thought it should be on the web someware.

#### grainde

##### Well-known member
Ok havent got much time but how about taking 2 lasers and shining one up directly into the sky ie at 90 degrees to the surface.(I know youd have to make sure it is 90 degrees...) Then take a second laser and point it to where the beam of the first stops or where you percieve it to. Measuring the distance between the two lasers and calculating the angle between them (theta) you can then use tan(theta) = opp/adj to calc the length of the opp ie percieved beam length. You could then repeat this calc for different distances and thetas between the lasers and see how opp changes.

You cant use distance points ie space stations etc as a reference, because they then change the measurement or percieved distance....(heisenberg and observer effect)

Not actually recommending this due to risk for aircraft...just putting it out there as an idea for calcing percieved distance of beam.

#### nouthyella

##### New member
Regarding cyparagon's suggested experiment that somebody take a long exposure to test the PBL hypothesis, I have found the following data:

1: Keck Observatory Laser Guide Star
http://keckobservatory.org/images/blog/Cooper-2-lasers-zenith-1.jpg

Unfortunately, due to the uploader of this image having scrubbed the EXIF data, I could not tell the field of view of this shot and hence was unable to triangulate the approximate height the two enhanced bright spots were. It is possible that these were the ~90km sodium layer, in which case the laser beams would be well past the PBL.
The two lasers were pointed at the zenith, and were approximately 70m apart.
Bing Maps - Driving Directions, Traffic and Road Conditions

2: GeMS Observatory's LGS
http://4.bp.blogspot.com/-payJ4tffYyM/TwcVonAgHbI/AAAAAAAAGx0/un9yD2hchcY/s1600/gs_mcao_lgs_gems.jpg

Astronomy Cmarchesin: Revolutionary Instrument Propels Astronomical Imaging to New Extremes

This LGS shot appears to show the five glowing spots ahead of the laser beam, while the laser has stopped being visible. If the dots are in fact images of excited sodium atoms glowing from being hit by the laser, this would disprove the hypothesis that the laser beam stops being visible due to angular separation. As of now I am unable to confirm or deny this.

Assuming that the calculator at http://www.astro.shoregalaxy.com/dslr_calc.htm is valid, and I input the values given on the article (Canon EOS350D, 2800mm telescope FL), I get 0.47 arcseconds per pixel. Since the horizontal distance from the camera/scope to the laser was not stated, it is not possible to calculate the height of the dots.

3: Another shot of the GeMS LGS:
http://www.gemini.edu/node/128

In this image, the LGS excited sodium spot is clearly circled, hence giving more weight to the hypothesis that the five points in the previous picture are the actual LGS sodium spots.

4: China's WIPM LGS:
http://english.wipm.cas.cn/rh/rp/201011/t20101130_62247.html
This also clearly states that the LGS sodium spot can be imaged from the ground. The two labels say "Na (sodium) LGS)" and "Raleigh scattering" respectively.

From the pictures shown, I believe that we can conclude to within a large degree of certainty that the laser beam is rendered invisible mainly due to the reduction of aerosol particles above the PBL, since the excited sodium spot of the LGS presents a target to render the beam visible, and the beam does indeed project to that point.

I believe that actual testing with a handheld power level laser to see the beam post-PBL would be impractical, considering the small angular separation between the PBL and ~90km, as shown in the pictures linked above. It will be near impossible to image the beam post-PBL due to its relative faintness. The beam could not be reliably imaged post-PBL even with 589nm lasers of >10w output, and only the patch of excited sodium at 90km rendered the laser beam itself visible.

Regarding cyparagon's post
Again I'll say: That would mean pointing at the horizon would make the beam much longer. That is not the case.
It possibly does mean that pointing it at the horizon makes the beam much longer, but you are unable to perceive the increase in length, due to the exact reason you mentioned yourself: the angular separation of the start and end of the beam does not change significantly when viewed, so a 100m beam looks almost the same as a 1km beam.

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#### snowboycosmos

##### New member
E- often when you see lasers advertised
In the description it sometimes says
Distance:-15 miles. Or Distance:120 miles
Do stronger laserbeams beam
further?
Maybe someone can show us a photo
Of three good lasers of differing strengths
And color pointing up at night...
Some solid visual evidence
Would uphold all this
Most interesting
scientific speculation.

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#### lasersbee

##### Well-known member
- often when you see lasers advertised
In the description it sometimes says
Distance:-15 miles. Or Distance:120 miles
Do stronger laserbeams beam
further?
Maybe someone can show us a photo
Of three good lasers of differing strengths
And color pointing up at night...
Some solid visual evidence
instead of all this clutter of scientific
speculation.
Good Idea.... Why don't you do that practical test
and report back to us with your pics since you seem
to define the Scientific fact/speculation on this Thread
as clutter....

BTW... if you believe the distance marketing hype
when buying a Laser you will need to to some
research.. Just saying...:beer:

Jerry

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

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#### Atomicrox

##### Well-known member
I've read both links and IMHO the PBL explanation should be taken with a grain of salt.
Why? Because both sites seem to have a significant amount of errors. Here's one from each, just to exemplify:

@ Laser Pointer Safety - Why laser beams outdoors seem to end
-"For a laser beam to be visible, some of the light must reflect off particles such as dust, smoke or water vapor."
According to wikipeida "Rayleigh scattering primarily occurs through light's interaction with air molecules." It'd still be there even if the air was 100% N2.

@ RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers
-"If the light output is only 5 milliwatts, why are people concerned about blinding other people?
The danger arises from the fact that the 5 milliwatts is concentrated in a very thin coherent light beam, and that the beam travels virtually unchanged for many miles."
Not true. Even a good quality beam with 1mRad divergence and 1mm diameter would be a 1m "dot" after only 1km.

I didn't dig deep but seems the NASA data is about aerosol causing IR scattering, not visible light. The same page says it has to do with wavelength specific absorption lines.
The doctor's quote also talks about an enhanced scattering on the PBL. I've never noticed beams getting significantly brighter before disappearing.
If I had to guess I'd say the quote is out of context.

There's a very simple experiment to decide this for once and for all - shine a laser from an airplane above 2km! If there's a beam than the PBL theory is out

#### Cyparagon

##### Well-known member
Thanks for the links, nouthyella. In reference to this picture, if they're 70 meters apart, the viewer is about 35 meters from each. Even at 100 feet from the source, the vast majority of the apparent beam only accounts for 300/90,000 = 0.3% of the actual beam length.

None of that would be visible if you were right next to the source.

For the people that say the PBL is the primary reason the beam stops, I'm curious to know what you think: are you suggesting the beam wouldn't stop if the PBL was infinite? If you pointed the laser straight up, where would it go? To the horizon?:can:

#### nouthyella

##### New member
Thanks for the links, nouthyella. In reference to this picture, if they're 70 meters apart, the viewer is about 35 meters from each. Even at 100 feet from the source, the vast majority of the apparent beam only accounts for 300/90,000 = 0.3% of the actual beam length.

None of that would be visible if you were right next to the source.
Fully agreed. That is why I disagree that the angular separation is the primary reason the beam stops, being that one of your justifications is that you cannot see the beam become longer, and the counter to that justification has been explained in my post above.

For the people that say the PBL is the primary reason the beam stops, I'm curious to know what you think: are you suggesting the beam wouldn't stop if the PBL was infinite? If you pointed the laser straight up, where would it go? To the horizon?:can:
I suggest that the beam will stop at the zenith, if you pointed it straight up, assuming there is an infinite PBL, and assuming that the returning photons are not scattered by the PBL to the point of invisibility.

My argument is that the visibility of the beam sharply drops past the PBL, which renders it essentially invisible (hence stopping) before it reaches the zenith.

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#### grainde

##### Well-known member
None of that would be visible if you were right next to the source.

For the people that say the PBL is the primary reason the beam stops, I'm curious to know what you think: are you suggesting the beam wouldn't stop if the PBL was infinite? If you pointed the laser straight up, where would it go? To the horizon?:can:
It cant go to the horizon, because if the beam is going staight up, eg normal to the surface, the concept of horizon is meaningless.

So what about my suggestion of using 2 lasers to determine if the beam terminates at the same distance eg at the PBL (post 52)...If you did 1 or 2 measurements at the source (or a few meters away) and a couple of measurements, a mile or 2 away, you could compare the calculated perceived distances between the two different observation points....If the beam appears to stop at the same point ie both calculated distances are the same, you have your answer...:beer:

Edit: Maybe a mile is too much, cos you'd have to take the curvature of the earth into account for the pythagoras calc...maybe 0.5 miles would be better...

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#### Atomicrox

##### Well-known member
So what about my suggestion of using 2 lasers to determine if the beam terminates at the same distance eg at the PBL (post 52)...If you did 1 or 2 measurements at the source (or a few meters away) and a couple of measurements, a mile or 2 away, you could compare the calculated perceived distances between the two different observation points....If the beam appears to stop at the same point ie both calculated distances are the same, you have your answer...:beer:
You wouldn't be able to see a beam pointed at the zenith from 1 mile away unless you have some star wars grade shit

I still think someone should perform the airplane experiment, it's way easier

#### grainde

##### Well-known member
You wouldn't be able to see a beam pointed at the zenith from 1 mile away unless you have some star wars grade shit
Ok I cant comment on that, but I already updated the distance to minimise the curvature of the earth You could always use a shorter distance too!

In any case a single measurement from the source would give you the distance to the perceived termination point of the beam and this could be compared with the known distance of the PBL!

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#### Cyparagon

##### Well-known member
I disagree that the angular separation is the primary reason the beam stops
I think there might be a confusion between "why the beam physically stops," and "why the beam appears to stop."

Obviously, there cannot be a beam if there is little or no material to scatter the light. I think we can all agree that the beam physically stops when the atmosphere stops and/or where the PBL stops.

The point I'm making is the reason why it appears to stop (from the perspective of the pointer user) is the viewing angle - the PBL inherently plays no role in appearance if you cannot even see it. The beam would still appear to stop even if the PBL wasn't present.

#### Trevor

##### Well-known member
For those arguing for the aerosol theory, I have a riddle for you:

Say you have a laser that produces a visible beam, even in a vacuum. From your point of view, it would produce an infinitely long green laser beam. From your perspective holding the laser, what would it look like? Would you see it stop?

Trevor

#### Blord

##### New member
For those arguing for the aerosol theory, I have a riddle for you:

Say you have a laser that produces a visible beam, even in a vacuum. From your point of view, it would produce an infinitely long green laser beam. From your perspective holding the laser, what would it look like? Would you see it stop?

Trevor
The beam will stop in the vanishing point.

#### snowboycosmos

##### New member
"Visit the Apache Point observatory, they have a laser, which beam does not only go to the moon, but does also come back when it hits a mirror on the moon.

The APOLLO laser has just 2.3W power, but the less than 100 picoseconds short pulses have the radiant power of a 1.150 GW laser firing a continuous beam. The laser is a commercial product, nothing too special in power, you can buy such strong lasers if you are willed to invest large amounts of money."

I tried firing my green through my telescope
The beam did look wider(15cm)but not any longer
And it lost alittle assemblage .

#### SOG

##### New member
I read most of what you say, I haven't read the extra links.. but I think it should be easy to test out... I don't have time these few days, but I will try to do this test on weekend...

everyone got a digital camera? it certainly can suck more light in with it's CCD/CMOS when we do long exposure... so we can try to take pictures with different exposures and ISO.

So we can tell if it's our eyes... for iso 3200 with 1min of long exposures, or even 5min exposures can see even the lowest lights... well maybe not the lowest lights, but at least long exposures with high ISO can suck more light in than our eyes.

and we can try to use different ISO too, as I think that would make quite a different... so we can try to put different color lasers with different power... under the same fixed location, and try to use a camera(s) to take pictures with different ISO and exposures, fixed at near point first, and then later move further away from the lasers as to test out and see if the angle does make a different... I think it's much easier that way to test the vantage point, perspective, trigonometry... whatever what some pictures. I think that is the cheapest equipment we have that can test this out... at least it's better than
just using our eyes to see it.

Anyone have time for this test? I would like to, but I don't have my Triport another Equipments with me...

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