why do Laserbeams stop "suddenly"

[Mannitu78]

hello, ive asked myself a couple of times, why my Laser has such a short beam....the strongest one i have is a 1500mw Blue Laser, now i cant say how long the beam is, might be 2km or only 500m, its hard to tell...in any case, the beam suddenly stops, it doesnt get weaker or thinner, its bright all the way through and then it looks like someone "cut" it off. Is this the nature of any Laserbeam?

 

[Alaskan]

It's called the light saber effect, you can adjust the length of the beam using a special spacial driver. LOL, sorry, a joke. It just appears to end due to various factors and also because of the perspective, that last few percent of the beam end you see is really a huge distance. You can google this same question and find answers.

I just used google to find a thread on it right here in this forum:

Why does the beam end in the sky?

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...

laserpointerforums.com

 

[gazer101]

Laser beams never end, they just keep on going until they get absorbed by something. Fun fact; by the time an average divergence laser pointer beam reaches the moon, it would have expanded to the size of Texas

 

[Alaskan]

Or to the sun, roughly 10% its diameter.

 

[Encap]

hello, ive asked myself a couple of times, why my Laser has such a short beam....the strongest one i have is a 1500mw Blue Laser, now i cant say how long the beam is, might be 2km or only 500m, its hard to tell...in any case, the beam suddenly stops, it doesnt get weaker or thinner, its bright all the way through and then it looks like someone "cut" it off. Is this the nature of any Laserbeam?

May be useful for you to understand why you see a laser "beam" at all/ever.
I will try to steering you towards reality of laser beam visibility --here goes.

The fundamental reality of "beam" visibility as follows:
It is not a matter of output power or wavelength.
A "laser beam" is invisible in a vacuum regardless of wavelength or power,

Beam visability to an observer is complicated real world phenomenon of light, eye sensitivity, and reflection from particle/aerosols in the air the details of which are not intuitively obvious.

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.

"In a vacuum, the laser beam itself would be invisible - regardless of power or color. As a laser beam passes through Earth's Atmosphere some of the photons encounter large airborne particles which reflect some of the light back to an observer. This only creates intermittent tiny bright flashes of light or "knots" in the beam - it is not why we can see the beam itself.

It is extremely small airborne particles called aerosols having a diameter significantly less than the wavelength of the light that causes the beam to become visible.

The effect of minute particles scattering light is called Rayleigh scattering and it's most noticeable effect is to turn the daytime sky blue. Rayleigh scattering causes photons to be scattered in a roughly spherical manner around these particles. Some of the light is scattered forward (in the direction of the beam), a lesser amount is scattered to the sides and about the same amount that is scattered forward is scattered backwards towards the light source. This backwards scattering is why the beam is more visible to people standing near the astronomer using it, than people standing some distance to the side. The more of these minute particles there are in the atmosphere, the mo re Rayleigh scattering there is."
From :RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers http://calgary.rasc.ca/atmosphere.htm

Really you need to understand aerosols in air --is very interesting actually--you need to know what they are locally and density of same to be able to figure laser beam visibility possibilities of one place compared to another place on the earths surface. Example: the most aerosol-laden air in the United States today pales in comparison to Asia. So laser beams are more visible there, generally speaking. Depending on the season and weather conditions, surges of aerosols can make their way into the atmosphere almost anywhere on Earth

See NASA Earth Observatory page for an excellent explanation of aerosols in air with lots of pictures and charts of the planetary distribution of aerosols : Aerosols: Tiny Particles, Big Impact : Feature Articles

 

[steve001]

It does not. Goggle "Sam's Laser FAQ". It has tons of info.

 

[RB astro]

To understand why laser beams just stop suddenly, you have to watch the movie, "The Truman Show"...

 

[Unown (WILD)]

To understand why laser beams just stop suddenly, you have to watch the movie, "The Truman Show"...

OMG I love that movie

 

[Cyparagon]

Why do these railroad tracks just end suddenly so far off in the distance?

Familiarize yourself with the concepts of horizon and vanishing points, and this is really easy to understand.

 

[Edgeoftomorrow011]

Clouds are things too and you can't see them at night

 

[skijohn]

Why do these railroad tracks just end suddenly so far off in the distance?

Familiarize yourself with the concepts of horizon and vanishing points, and this is really easy to understand.

This is the best description and illustration I've seen of what happens...