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In various works sci-fi writing, submarines and underwater vehicles/installations are said to be armed with laser weaponry that specifically fire blue-green beams- would such lasers have any actual advantage over, say, red or yellow laser beams (such as range, damage output, etc.)?

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    $\begingroup$ Lasers under (liquid) water as a weapon is quite frankly stupid. If you look at the US ATL project, one of their major technical problems was the water in the humidity of the air. Liquid water has much much more water in it than air does (in chemistry they often assume it to be close enough to 100%). $\endgroup$ – Aron Mar 23 '16 at 0:50
  • $\begingroup$ It may be a bad idea, that doesn't mean it's a bad question though. $\endgroup$ – Tim B Mar 23 '16 at 10:15
  • $\begingroup$ I second that. Why would he even ask, if he knew the answer already? $\endgroup$ – user6415 Mar 23 '16 at 11:44
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I don't think lasers would work well underwater.

Lasers are focused light, and also happen to generate a lot of heat.

Water distorts light, and as such isn't a good medium for lasers to travel though (you might use one for cutting at a close-ish distance, but they wouldn't work too well as weapons).

Furthermore, the heat generated by said laser would cause water to heat up (it would thus lose energy very quickly), and also boil, which would further upset the path of the laser and cause it to diffuse.

The second answer on this thread really, really explains why this wouldn't work: https://scifi.stackexchange.com/questions/13812/can-laser-weapons-work-under-water

Red vs Blue

Now as far as the color of the laser is concerned, blue and green ones operate on a lower wavelength, and are more "powerful" than red ones. (this is a bit simplistic, I will try to update later, other people can feel free to comment-correct me about this)

Note: I was thinking about this question from a technical POV and simply answered, but the more I think about it, the less WB related it is, although I'm not sure what I would quote as a close reason, as nothing seems to really "fit" except that it's more of a physics question than a WB one. OP, maybe try editing it a bit to make it about WB?

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  • $\begingroup$ The energy of the individual light particles (photons) goes up as the wavelength decreases, so a single green photon has greater energy than a single red photon, but the power of the laser also depends on the total number of photons it's emitting per second, so you can have a red laser that's more powerful than a green one. Another issue though is that the shorter-frequency photons will be able to penetrate further into many materials, so if you want to do some damage short frequencies are better in that sense, and it might also relate to how quickly the laser would get defocused in water. $\endgroup$ – Hypnosifl Mar 22 '16 at 23:32
  • $\begingroup$ Just as an extention, this really only works in clear clear water. A little bit of murky and these fail quickly. Power depends on watts and has not to to do with frequency which is colour $\endgroup$ – Twelfth Mar 22 '16 at 23:36
  • $\begingroup$ Correction: just noticed I accidentally said "shorter-frequency photons" and "short frequencies" in my above comment when I meant to say "wavelength", not "frequency"--shorter wavelength corresponds to higher frequency. $\endgroup$ – Hypnosifl Mar 23 '16 at 13:02
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The color of a laser will make some difference in terms of underwater usage. Take a look at this page:

https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

And these images:

Absorption vs wavelength Absorption vs wavelength, visible spectrum

You can see that visible light is already the least absorbed wavelength in water, so anything else is going to be worse. Blue is actually near the least-absorbed wavelength so would be the best choice.

However even though it is the best choice it is still a terrible one, water is going to scatter and absorb your laser all over the place.

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