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Whenever movies portray laser weapons, they do so with a very visible beam hitting the target and then incapacitating and/or killing them. Laser weapons exist in real life, although only at the prototype stage. See this Wired story for instance.

Is there any truth to laser weapons having a visible beam when fired? Or is it just for entertainment purposes?

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    only lasers which fire a beam in the visible light spectrum would leave any kind of visible trail and that only in a participating medium (fog) which would lessen the intensity of the laser on the target. a US military device exists for igniting roadside explosives but that doesn't leave a visible track – ratchet freak Feb 9 '12 at 3:05
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    There is a very funny SciFi book called Who Goes Here where the laser-armed forces of earth encounter a rebel army using traditional weapons in an fire-prone environment. The battlefield ends up very smoky, so much so that the laser weapons beams are dissipated by the smoke making them mostly ineffective, causing irritation rather then death in the enemy. This is a real problem with high-powered laser weapons: when they are visible, they are also useless. – matt_black Feb 9 '12 at 21:52
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    A laser powerful enough to cause physical harm to a target would surely be able to ionize air. And if I recall correctly, heating a gas changes its index of refraction, as evident in the heat lines over highways on a hot day. The difference in the index of refraction of the ionized air and the surrounding air could cause the light to scatter enough to be visible, but the actual beam of light would have to be contained within the column of ionized air. So, yes, you should be able to see these lasers, but they fire in extremely quick pulses, and it takes time for a laser to ionize air, with an e – user11770 Mar 3 '13 at 21:35
  • @jumbojimbolaya: I think when it changes the index of refraction it causes the beam to be more focused, not scattered: en.wikipedia.org/wiki/Filament_propagation – endolith Mar 4 '13 at 19:32
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Currently researched high-energy laser weaponry (the MIRACL laser, the Pulsed Energy Projectile and the Tactical High Energy Lasers) use deuterium fluoride lasers, which operate at the wavelength of 3800 nm. Visible light has wavelength range of 390 to 750 nm.

Even if the laser would use visible light, the beam would only be visible in medium that would partially reflect the light. As for example water particles (clouds, rain, fog) or smoke.

US Navy testing their THEL (note how the beam is not visible):

http://www.youtube.com/watch?v=XyTRhw8qmHE

Also, from usefulness point of view, especially for man portable weapon, you don't want visible beam, as that would put shooter in tactical disadvantage. And indeed, the article you refer to talks about goals:

As a lethal system, a laser sniper rifle would be a formidable weapon: perfect accuracy at any a range measured in miles, with no windage, no drop, and no need to allow for target movement. And it would not give the firer’s position away with a report, smoke or muzzle flash – all the enemy would see would be the effect when it hits the target.

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    While we are playing hypotheticals, @JoãoPortela, I would give the gunner a video screen that super-imposed the reflected light in the laser's wavelength as a red highlight over the normal (visible light) camera, so only the operator could see where the laser was striking. – Oddthinking Feb 9 '12 at 14:05
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    @JoãoPortela: tracers are used, because in case of bullet firing machine gun, you have bullet drop, bullet drift due movement of firing platform, bullet drift due wind. In case of laser you don't have any of that, it hits exactly where it's aimed at. – vartec Feb 9 '12 at 14:13
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    @JoãoPortela: why not? that's what sight are for. And there is no bullet travel time either. Anyway, if you'd really want, you could have auxiliary spotting laser with on/off switch. No point in making main one visible. – vartec Feb 9 '12 at 16:56
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    A smoke generator that emits fluors sensitive to popular laser wavelengths will probably hit the battlefield shortly after man-portable laser weapons: en.wikipedia.org/wiki/DyLight_Fluor Introduction of small lasers faces legal problems as well: en.wikipedia.org/wiki/Protocol_on_Blinding_Laser_Weapons – user951 Feb 9 '12 at 20:34
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    Just wanted to point out the lasers only go in perfectly straight lines in theory. They are light, so are subject to refraction. – Benjol Feb 10 '12 at 8:02
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As @Vartec answered, laser weapons are probably not likely to use the visible spectrum. But lasers of the appropriate wavelength are visible to the naked eye even without fog due to Rayleigh Scattering. This is an image of a laser guide star (used to correct for atmospheric distortion) propagating from the Gemini North facility in Hawaii. It is (easily) naked-eye visible : laser guide star beam propagated from Gemini North facility

Lasers don't have to be particularly powerful to be naked-eye visible. Consumer-grade green lasers are visible and are commonly used by amateur astronomers to point out particular locations in the sky.

  • "Consumer-grade green lasers are visible" most things sold to consumers as lasers aren't, they're merely LEDs with a collimator lens in front of them. Lasers being illegal for consumers to own in many countries is one reason for that. – jwenting Feb 10 '12 at 7:08
  • doesn't Rayleigh Scattering require objects (gases) of various refractive indices? – vartec Feb 10 '12 at 15:07
  • I'm not a physicist or laser technician, but I believe that the normal fluidity of air is enough to cause scattering. Maybe in a still room that wouldn't be the case. – Larry OBrien Feb 10 '12 at 17:37
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    @jwenting I'm not sure about "most things sold to consumers" or outside the US, but inside the US, red (650nm) and green (532nm) lasers of a few milliwatts power are legal and inexpensive. (Other wavelengths are available but more expensive.) – Larry OBrien Feb 10 '12 at 17:43
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    Clearly, there's only one way forward skeptics.stackexchange.com/questions/8011/… ... I assure you that the Meade device, if I'm correct in believing it's a laser, projects a beam that is naked-eye visible for at least 100 yards. – Larry OBrien Feb 13 '12 at 1:28

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