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01-24-2017, 01:20 PM #17
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Encap
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Re: Blue laser and the night sky ?

Quote:
 Originally Posted by Crazlaser Bad reasoning because if you point the laser horizontally at low mountains the beam fades off before it hits them. However as you move away from the axis of the beam the beam appears to extend until it touches the mountain. Everyone please consider this for a moment. If you are holding a light but infinitely long steel rod (don't ask me why is doesn't bend or how you can hold it) that doesn't bend, think about this, it would appear to end, right?
I think you are mixing are apples and oranges a bit. What members have been responding to is the photo of a blue laser point up in the sky and the question way does the beam stop aburptly.

We do not see in length, we see in angles. That is why objects farther away look smaller - they take up less angle so an observers position relative to a beam has a lot to do with how far a beam appears to go--field of view and trigonometry problem. That is a different sort of problem and answer --your steel rod analogy and dot on a mountain.

The physical question as to why a beam is seen at all when pointed up in the sky and why it appears to stop is the one people have been discussing here.

There is no visible beam from a laser of any power 1mW or 1MW in a vacuum.
Think about why you see a laser beam at all, ever? You don't see a laser beam, you just see reflections from particles in the air the beam hits. A laser dot is a different story entirely.

Whether or not you can see the beam or the extent to which you can see a beam depends upon many things -- among them the position of the observer in relation to the source of the beam, amount of aerosols in the air of the Planetary Boundary Layer reflecting light to an observer, an individuals eye sensitivity to any given wavelength.

At sea-level, one cubic inch (1 inch x 1 inch x 1 inch) (16.39 cm3) of "air" contains approximately 400 billion billion (4*1020) air molecules, each moving at about 1600 km/hr (1000 miles/hr), and colliding with other molecules and anything else they come into contact with about 5 billion times per second. This is the reason for "air pressure". The amount of particles in that air that can reflect a portion of a laser beam's light back to your eye determines if you can see it or not.

It all depends upon atmospheric conditions--a beam you can see extremely well in fog or area with high concentration of particulate matter in the air can be almost invisible in clean clear air

Laser beam visibility is highly dependent on ever changing atmospheric conditions and aerosols in the air.
You never actually see the laser beam --what you see is the reflections from particles in the air. A laser dot is a different story.

Last edited by Encap; 01-24-2017 at 10:07 PM.

01-24-2017, 09:02 PM #18
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Accutronitis
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Re: Blue laser and the night sky ?

The Planetary Boundary Layer ! I think I get it now, Thanks guys.......

01-25-2017, 12:10 AM #19
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Crazlaser
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Re: Blue laser and the night sky ?

No it cannot be the planetary boundary layer. That is absolute bullcrap. Explain why when I point the laser at a horizontal angle to the ground, at a mountain, the beam appears to fade before the mountain. It's not going up, up and away or something. It's staying below 50 feet above the ground. I call bull on that theory.^
I edited this image provided by laserpointersafety.com

"But above the Planetary Boundary Layer, air is much cleaner. After the beam exits the PBL and enters cleaner air (fewer aerosols), much less light is reflected back. The beam seems to disappear."

Then explain why this boundary level is horizontal. As if 500 meters away it suddenly stops. And in all directions too. It's not just that it's foggy here and clear there. It's a circle ending at the same distance in all directions.

BULLCRAP
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Last edited by Crazlaser; 01-25-2017 at 12:23 AM.

01-25-2017, 02:14 AM #20
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Re: Blue laser and the night sky ?

Quote:
 Originally Posted by Crazlaser No it cannot be the planetary boundary layer. That is absolute bullcrap. Explain why when I point the laser at a horizontal angle to the ground, at a mountain, the beam appears to fade before the mountain. It's not going up, up and away or something. It's staying below 50 feet above the ground. I call bull on that theory.^ I edited this image provided by laserpointersafety.com "But above the Planetary Boundary Layer, air is much cleaner. After the beam exits the PBL and enters cleaner air (fewer aerosols), much less light is reflected back. The beam seems to disappear." Then explain why this boundary level is horizontal. As if 500 meters away it suddenly stops. And in all directions too. It's not just that it's foggy here and clear there. It's a circle ending at the same distance in all directions. BULLCRAP
That a good point, if I start out shooting my laser straight up it looks like it stops a certain distance away but if I then swing the beam down to where it just clears the houses in my neighborhood it still looks like it stops the same distance away as it did when it was straight up ?

01-25-2017, 01:54 PM #21
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Crazlaser
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Re: Blue laser and the night sky ?

Well I've tried severely unfocused a 3 watt 445 and the super fat beam is visible to the same distance as the condensed beam.
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445nm 2.9 watt Sanwu Silver Series
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532nm 70mw Pman
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635nm 800mw Sanwu Challenger multimode
Laserbtb 980nm 800mw
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01-25-2017, 06:51 PM #22
 Super Moderator Join Date: Sep 2013 Location: Morro Bay, CA, USA Posts: 8,170 Rep Power: 977216
Hap
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Re: Blue laser and the night sky ?

Quote:
 Originally Posted by Crazlaser It has to appear to end some time right? It's like an asymptotic type thing. The closer you are to the axis of the beam the end of the beam compresses closer and closer. Imagine drawing it out with your mind. Otherwise the beam would appear to bend. But it's still viewable, right?
Well sorry. I'll just shut my mouth from now on.

-Alex
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01-25-2017, 08:22 PM #23
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diachi
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Re: Blue laser and the night sky ?

Quote:
 Originally Posted by Hap Well sorry. I'll just shut my mouth from now on. -Alex

Not sure how your post relates to the quote?
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01-25-2017, 08:48 PM #24
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Accutronitis
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Re: Blue laser and the night sky ?

Quote:
That all make prefect sense, I always knew from the moment I asked the question it had to be some sort of optical illusion but I didn't know how or why it was happening and I'm a big fan of the how and why of things. Thanks.....

Last edited by Accutronitis; 01-25-2017 at 08:49 PM.

01-25-2017, 08:58 PM #25
 Class 2M Laser Join Date: Oct 2016 Location: Maryland, USA Posts: 813 Rep Power: 18863
Crazlaser
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Re: Blue laser and the night sky ?

Quote:
 Originally Posted by diachi Not sure how your post relates to the quote?
I told him that he was wrong about resizing the picture.
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Welcome

My lasers
Sanwu 405nm 250mw Sanwu Pocket Series
445nm 1+ watt china generic laser (broken host)
445nm 2.9 watt Sanwu Silver Series
450nm 3mw LP450-1B
532nm 50mw laserbtb
532nm 301
532nm 70mw Pman
532nm 92mw Pman
635nm 800mw Sanwu Challenger multimode
Laserbtb 980nm 800mw
Eagle Pair goggles 190-540nm & 800-2000nm OD4
Crazlaser's Collection

01-25-2017, 09:31 PM #26
 Super Moderator Join Date: Sep 2013 Location: Morro Bay, CA, USA Posts: 8,170 Rep Power: 977216
Hap
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Re: Blue laser and the night sky ?

I apologize

-Alex
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"Open your eyes, look within. Are you satisfied with the life you're living?" -Bob Marley
"The stars should fall and the earth be riddled, imagine all your problems be seem so little, funny how the world so brittle." -Tribal Seeds
“The universe is a pretty big place. If it's just us, seems like an awful waste of space.” -Carl Sagan
You are important: Pale Blue Dot

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