Laser Distance
Monday, July 4th, 2011 |
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Do lasers travel for an infinite distance? If I shine a 1mW laser at a 70 mile wall in clear air will I see it?
Will I see the laser light on the wall from 70 miles away if the air is clear inbetween the laser and the wall and if there is nothing blocking the laser from shining at the wall even if the laser is 1mW?
oldprof: Will I see anything with my 1 mW laser at 2 kilometers away?
Good thought evoking question.
The answer is, of course, no they don't go on forever. But that's simply based on the 2nd Law of Thermodynamics that says all useful energy, like E = hf, will eventually become useless entropic energy. So perhaps billions of years from now that laser photonic energy will die out when the photons do.
But having said that, they will go a long way barring interdiction by anything in their path. But your coherent beam isn't 100% coherent; so some of it will slough off en route to wherever your target is.
Thus all the brightness that left your laser gun will not arrive at the same brightness on the wall. Brightness is determined by the number of photons per unit time; so as some photons escape the beam, the beam's brightness diminishes the farther out it goes from the source.
Then there's that "clear air" you presume. Unfortunately, there ain't no such animal.
There is always dust, moisture, pollen, etc. in the air. So your laser photons will need to push through those bits and pieces like a pin ball needs to go around the pins of a pin ball machine.
Frankly, some of those poor little guys are doomed. They will ricochet and go off into the boondocks somewhere. They will be absorbed and kept or re-emitted at some different, probably lower, energy level. And the more stuff there is in the air, the least likely your laser beam will be bright enough to see on the wall 70 mi away.
Finally, 1 milliWatt is not much power at the source. So the number of photons emitted per unit time is miniscule and the consequent brightness will not be... bright that is. So you are starting with a dim bulb in the attic. So unless you have some special light enhancing observation device, like night vision goggles, you are not likely to see anything at 70 mi. Sorry.
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