[thedoc (OP)]

asked the Naked Scientists:
   Hi Chris and all the team, How do you stop a laser beam in its tracks?

At a recent light show in Alice Springs I was again perplexed by laser beams, not shining out to infinity and beyond, but of distinct finite lengths.

The only vaguely related science snippet I can think of is for optical telescopes that shoot a laser into the troposphere? and from its reflected characteristics the telescope is programmed to adjust its mirrors profile to account for atmospheric disturbance

Could you explain how A and B above work please

See you in The Alice sometime:)

Fred Richardson
What do you think?

[Janus]

asked the Naked Scientists:
   Hi Chris and all the team, How do you stop a laser beam in its tracks?

At a recent light show in Alice Springs I was again perplexed by laser beams, not shining out to infinity and beyond, but of distinct finite lengths.

The only vaguely related science snippet I can think of is for optical telescopes that shoot a laser into the troposphere? and from its reflected characteristics the telescope is programmed to adjust its mirrors profile to account for atmospheric disturbance

Could you explain how A and B above work please

See you in The Alice sometime:)

Fred Richardson
What do you think?

With a laser light show you can only see the beams when they interact with a medium which reflects some of the light to your eye.  This is usually provided by a "haze machine" which projects fine liquid droplets into the volume around them.  The haze is generally too light to be seen until illuminated by the lasers.  The reason you see the lasers appear to end after a certain distance is not because the lasers have a limited range, but because the volume that the haze machines can effectively fill is limited.

If you have access to a laser pointer, you can demonstrate this yourself.  In a darkened room, turn on the pointer. You will likely not see the beam of the laser (unless the air in the room is particularly dusty.)  Now take an aerosol can (like an air freshener) and spray it into the path of the beam.  You will suddenly see the beam as the light reflects off of the small aerosol droplets.   It is basically the same thing with laser shows.

[evan_au]

It is true that haze from smoke machines, or dust swept up from the ground only extend to a certain distance above the ground.

(A) But there is another effect at work here: Perspective.
- If you shine a laser beam at a particular star (for example), an observer right behind the beam will see a very short-looking beam that ends on the star.
- If you are standing off to one side, you will see a much longer-looking beam that extends part way across the sky, and ends somewhere near that star.
- In perspective, the star is the vanishing point for the laser beam; how much of the sky it appears to cross (from your viewpoint) depends on the angle you see between the laser pointer and the star.
- So the laser beam does not "stop", but it asymptotically approaches the target star, from your viewpoint.

(B) Astronomical telescopes with deformable mirrors are able to partially counteract the heat haze in the atmosphere, which makes stars "wobble around", producing blurry images. They do this by monitoring the distortion of a "guide star"; if no guide star is available, they can use a laser beam to produce a "laser guide star".

One form of this is to use a laser beam with the right wavelength to excite sodium atoms in the upper atmosphere. The astronomers can detect the light subsequently emitted by the sodium atoms to estimate the distortion in the atmosphere.

(C) There is a very real  sense in which physicists can "stop" laser light, or slow it down to a walking pace.
By using a laser of just the right wavelength, and by preparing a gas with other laser beams to put the electrons in the correct state, they can produce material where the speed of light for one particular wavelength is only a walking pace. Similar effects can be achieved with materials having a negative index of refraction.

See: https://en.wikipedia.org/wiki/Slow_light

[jeffreyH]

It is true that haze from smoke machines, or dust swept up from the ground only extend to a certain distance Uh above the ground.

(A) But there is another effect at work here: Perspective.
- If you shine a laser beam at a particular star (for example), an observer right behind the beam will see a very short-looking beam that ends on the star.
- If you are standing off to one side, you will see a much longer-looking beam that extends part way across the sky, and ends somewhere near that star.
- In perspective, the star is the vanishing point for the laser beam; how much of the sky it appears to cross (from your viewpoint) depends on the angle you see between the laser pointer and the star.
- So the laser beam does not "stop", but it asymptotically approaches the target star, from your viewpoint.

(B) Astronomical telescopes with deformable mirrors are able to partially counteract the heat haze in the atmosphere, which makes stars "wobble around", producing blurry images. They do this by monitoring the distortion of a "guide star"; if no guide star is available, they can use a laser beam to produce a "laser guide star".

One form of this is to use a laser beam with the right wavelength to excite sodium atoms in the upper atmosphere. The astronomers can detect the light subsequently emitted by the sodium atoms to estimate the distortion in the atmosphere.

(C) There is a very real  sense in which physicists can "stop" laser light, or slow it down to a walking pace.
By using a laser of just the right wavelength, and by preparing a gas with other laser beams to put the electrons in the correct state, they can produce material where the speed of light for one particular wavelength is only a walking pace. Similar effects can be achieved with materials having a negative index of refraction.

See: https://en.wikipedia.org/wiki/Slow_light

You never cease to amaze. Wow!