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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 disturbanceCould you explain how A and B above work pleaseSee you in The Alice sometime:)Fred RichardsonWhat do you think?
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