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# Blue laser and the night sky ?

#### Encap

##### Well-known member
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.

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#### Accutronitis

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

#### Crazlaser

##### Active member
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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#### Accutronitis

##### Banned
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 ?

#### Crazlaser

##### Active member
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.

#### blueasthesky

##### Super Moderator
Staff member
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

#### diachi

##### Well-known member
Well sorry. I'll just shut my mouth from now on.

-Alex

Not sure how your post relates to the quote?

#### Accutronitis

##### Banned
Simple answer, we are living in a simulated reality and the program has set the limit of how far a beam can be seen before it disappears from view.... No problem, there's your answer. OK, fictional...

Have you considered the divergence causes the beam to expand so much you just can't see it as a beam anymore? Try that mountain experiment with a uber low divergence laser and see what happens. Our 1 mRad laser pointers have far too much divergence, of course this depends upon the distance. How far away is that mountain?

At 25,000 feet away, less than 5 miles, the beam diameter of a 1 mRad laser will be about 25 feet wide, I don't think you will see the beam when it is that wide from any of our 1 watt 532 nm green laser pointers. If your pointer has the more common 1.5 mRad divergence (which is a fairly good pointer) at 5 miles the beam will be approaching 40 feet wide. If your pointer uses a 520 nm laser diode, the divergence is typically far greater (unless using a beam expander) and at 5 miles the beam diameter will be over 60 feet wide. You just can't see the beam from your position when that wide, even if close to 1.5 watts of power. I'm not sure someone standing on top of a mountain five miles away could see the beam pass over them when having expanded to that diameter, if any, probably very little hint of it. Double the distance to 50,000 feet, or less than 10 miles and the diameters double. That combined with less particles at high altitude and we have no chance of seeing the beam extend into the far atmosphere, it will just appear to end.

Anyone here care to calculate how much distance is traveled in the last visual 1/10 of the beam from the perspective of the individual pointing the laser, as a percentage of the full distance traveled before the beam disappears? I think the figures would be interesting, I doubt our depth-impressions are what they appear to be, when viewed from the end of a rod or beam of light. i.e., if the last 10% of the length of a laser beam is 50 times further away than the first 1000 feet, does that make the length of what appears to be the last 1/10 fifty times more length of travel? I suspect the ratio is much greater than that.

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.....

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#### Crazlaser

##### Active member
Not sure how your post relates to the quote?

I told him that he was wrong about resizing the picture.

Staff member
I apologize

-Alex