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Can a laser or laser beam travel through water? I saw a battle scene in Star Wars: The Clone Wars, season 4, episodes 1 and 2 where they are exchanging laser blasters. Though I don't know anything about their lasers or what is special about that particular laser, I want to believe that it could work underwater, especially since it scatters when it bounces against crystals. Water is like thousands of crystals, if I may make a comparison.

Yet, in that same episode they used light sabers, that can heat up very quickly when they come into contact with anything. Shouldn't the water boil when it comes in contact with the light saber if that's the case?

Backing up to the idea that Lasers can't work underwater. In Gundam SEED, they can't use laser swords or laser blasters underwater, so they use a combat knife or a bazooka instead.

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    Technically Blasters aren't laser weapons, but are instead particle weapons. Instead of a coherent beam of light, the blaster fired a compressed, focused, high-energy particle-beam that is very destructive, commonly referred to as a "bolt". I am not sure what effect this would have on using it underwater. – Xantec Mar 27 '12 at 4:15
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    The weapons in all Gundam series are plasma type, none are lasers in any way. – SteveED Mar 27 '12 at 4:35
  • For lightsaber based discussions, go to this question: scifi.stackexchange.com/questions/10557/… – Lobo Mar 27 '12 at 10:30
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    Suggest this question be migrated to physics. – Wikis Mar 29 '12 at 13:37
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    I agree with @Wikis, this sounds like it was intended as a real-life physics question, and belongs over there... – Izkata May 27 '13 at 21:29
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Medium always influences the performance of a weapon but that's not a given - weapon designers can change the design to balance the strengths and weaknesses in a different way. Mind that you can never design the "perfect" weapon because there is always a trade off. Unfortunately, most authors aren't aware of this delicate balance or time constraints make it impossible to do it. So most weapons in SciFi movies and books simply don't make sense or are utterly unbalanced.

For Star Wars, blasters are used but it's not really clear what a "blaster" is other than that it can be set to different power levels dealing damange that ranges from "stun" to heavy explosions.

They do have some characteristics of a plasma weapon but they aren't. Plasma has a strong urge to disperse, so you would need something to keep it together (focused) and that something has to go with the plasma. There are plasma configurations which are self-stabilizing but we'll have to see what their limits are.

Also the amount of energy you can put into a plasma has an upper limit. In fact, it's way more simple to build a more powerful laser weapon but real lasers work completely different than what you can see in any movie. With the amount of power available to a Star Wars battle cruiser, you could easily build a multi-megawatt laser. That laser could cut through anything - other ships, moons, planets - in a reasonable amount of time and there would be no defense against it.

Shields couldn't stop it because the laser brings all it's energy into a very small place, so the whole surface of the shield must be able to withstand that amount in every spot. You could try to build a shield that can be "concentrated" but then I come with two lasers. If you could power such a shield, it would be a waste because you could use the same energy to power hundreds of lasers that simply blast any enemy out of the sky before they can fire on you making shield useless.

That would be disastrous to the story: All battles would be over in a few seconds. Whoever gets to fire first wins. No heroic deeds. Worse, battles would be fought over distances of 100'000km up so the enemy would be nothing but a blinking dot on a radar. If you looked out of a battle ship, you'd see nothing but black space. No camera angle would be able to catch the action.

So the whole discussion is about a technology that we don't understand and that the inventors never understood, either. It's a means to create tension as the story progresses, nothing more. Physics and logic as we understand it has to take a second seat behind suspense and the demands of the story.

  • A mirror designed for the correct frequency would reflect the laser beam, but of course a tunable or multifrequency laser can easily defeat one. Lasers of course don't work well under water or in an atmosphere, as the matter would absorb and disperse the beam (rl lasers do suffer from this, it limits their range and limits the choice of frequencies for them so as to minimise the effect, one reason the ABL as so large it required a B747 to carry it was the extra power needed to compensate for atmospheric dispersion/blooming). – jwenting May 28 '13 at 6:12
  • I did some research on this. The best laser type for atmosphere is a UV laser, it can pass through thousands of kilometers of air without losing much power. Just like UV light from the sun. And just like UV light, it would burn the skin of every living thing that has a direct line of sight to almost any point on the line of fire. So when fired from orbit, it would kill every living thing on the same side of the planet. As Sarah Connor put it: "Anyone without a Sunblock below 10'000'000 will have a really shitty day." – Aaron Digulla May 28 '13 at 7:10
  • It loses power, Aaron, because it loses focus. The energy gets dispersed as it is scattered. That's why such high power is needed for long distance shots, to ensure that at the aim point there's still a high enough energy density to heat the target to where damage ensues (in contrast to popular belief, laser weapons don't cause things to blow up or melt directly, they cause fuel and especially fuel vapour to overheat and ignite, possibly after burning a small hole). That same high power of course means that you get collateral damage over a large area, which is what Connor refers to. – jwenting May 28 '13 at 9:14
  • I know that you can't focus all the energy in a single spot and that it gets dispersed (the answer below gives all the gory details). As a weapon, a UV laser beats all other laser types when you want to slag ground targets from an orbit in terms of cost, efficiency and technology level. – Aaron Digulla May 28 '13 at 10:07
  • With regard to not being able to contain plasma: That is wrong. You can create plasma that produces its own magnetic field to contain itself - just extrapolate this experiment: spectrum.ieee.org/tech-talk/energy/nuclear/… – flq Jul 17 '13 at 14:44
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Light scattering is a relatively complicated process.

In short, the beam will get wider and lose power, making it work less well.

Both of these reduce the intensity. If you like, think of intensity as pressure, as both intensity and pressure are inversely related to area. That is to say if two beams have the same power but one has a wider spread and thus larger area, that laser will do less 'damage'. In the same way, a bullet which hits with the same force but has a larger cross section will do less damage.

Now on to Scattering:

Unfortunately water is not really like crystals, which diffracts light into patterns. Water and most liquids are what we call amorphous. Here is a picture of Water (amorphous) on the left and Ice (crystalline) on the right:

enter image description here

Basically the atoms in an amorphous liquid cause the light to bounce every which way, very little lines up and it gets scattered.

The main process that occurs in light scattering is Mie Scattering, but a decent approximation is Rayleigh Scattering.

enter image description here

Ignore everything there except the second bracket.

  • In Rayleigh Scattering, the attenuation (loss of power) is proportional to the inverse of the wavelength (lambda, which looks like an upside down Y) to the power of 4! That's right, have a short wavelength and the attentuation shoots up. If we double the wavelength we increase the attenuation by a factor of 16, if we triple it it goes up by a factor of 81!

That's all well and good, but what does it looks like? Well that depends on the size of the particles in the water. Unless the water is completely clear lots more scattering takes place.

Here is a comparison of scattering in clear water, and milky water. enter image description here

It enters from the right, get's scattered in the murky liquid then passes straight through the clear water. The pattern on the far left has a much bigger area around it where the light has spread out.

Here is another one where we can see how the light is spread out in the 'turbid' fluid, and it doesn't go that far. This is the widening and the loss of power I talked about at the start.

enter image description here

To understand how the different wavelengths interact with water and other particles, here is some data:

enter image description here

You can see, major scattering occurs from the particles in the water but not the water itself (other factors do play into it, and the difference between air and water does matter over long distances in water).

I would liken this scattering effect to turning the laser rifle into a much weaker sawn off laser shotgun. Not as powerful but with more spread.

In short: yes it will work, just not as well.

[Edit: as requested from wikipedia:

enter image description here

Absorption plays a much greater role in clear water, and is dependant on what vibrational modes the EM wave excites in the molecule. See here for more of an explanation. TL;DR if it's anything but visible light it's going to struggle to pass more than a few cm through the water.

  • Blasters don't fire electromagnetic waves, so the answer is meaningless. – Lobo Mar 27 '12 at 10:24
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    I know, but the question is still "Can Laser Weapons Work Under Water?" The examples given aren't lasers but I'm still trying to answer the question. – AncientSwordRage Mar 27 '12 at 10:38
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    @bitmask don't thank me, thank physics! – AncientSwordRage Mar 27 '12 at 11:22
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    @Pureferret: I have an ambivalent relation to physics. I love it for its beauty and sometimes hate it for struggling with understanding it. – bitmask Mar 27 '12 at 12:08
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    @Pureferret - That graph is all you need to understand it, though! It's far simpler than scattering, actually. transmittance = e^(-absorption coefficient * distance); absorbance = 1 - transmittance. – Rex Kerr Mar 27 '12 at 19:12
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Laser weapons can work through water but not very well. The reason is that, as you suggest, the light will be absorbed by the water as well as the target, which will boil the water and cause all sorts of other problems.

Pure water has an absorption coefficient of about 2*10-2/m, which means that the bulk of the power will be absorbed after traveling 50m through water--and that's if the laser is the optimal wavelength (blue-green). Turbid water or other colors will considerably shorten this distance. So underwater, a laser is at best a short-distance weapon.

Laser swords don't actually exist, so it's less clear how they would interact with water. Presumably they would boil the water, making them rather interesting to use, but depending on the postulated physics of the device, various other things could potentially happen. For example, anything using magnetic containment wouldn't work underwater because of the relatively high conductivity of water.

Various particle weapons would go through water little better than they would go through a wall; there are about the same number of atoms there to collide with. Anything that shoots "bolts" typically is supposed to be some sort of particle weapon (e.g. magnetically-confined plasma).

  • I think actually lasers would be very poor underwater, as they would boil the water, making it bubble, and so further disrupt the laser light. It might work to boil your enemy, but as even a close combat weapon, I feel it would be tricky to handle. – Schroedingers Cat Mar 27 '12 at 8:20
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    What shown in Season 4 Episode 1,2 was blaster. And, blasters don't fire electromagnetic waves, so there's no point of absorption coefficient – Lobo Mar 27 '12 at 10:28
  • @SachinShekhar - The question asked about "laser weapons", so that's what I devoted the bulk of my answer to. Blasters are not lasers, and hence I added something on "particle weapons" (which blasters are). – Rex Kerr Mar 27 '12 at 18:56
  • @SchroedingersCat - You can impart energy to your target instead of the water at a rate proportional to the total absorbance of the target divided by the absorbance of the water. If your target is only a meter away or so, that could put the vast majority of the energy into the target (so the water wouldn't boil, but the target would be heated by ~1000 degrees C). You could come up for a simple bound on total energy delivered in the beam at a given diameter to avoid boiling (in clear water). In turbid water, it could be arbitrarily bad (e.g. if you can't see the target, neither can the laser). – Rex Kerr Mar 27 '12 at 18:59
  • @rex - I suppose if you are in clear water, it might work, but if you are in combat with someone the chanced of the water being clear are low. So yes, I agree, they may work for stealth close weapons. But I think the predictability would become a limiting factor. Even 1M away in cloudy or churned water might be too much. And if someoe is trying to kill me with a laser, the water will be very churched up.... – Schroedingers Cat Mar 28 '12 at 8:03
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According to books signed in to star wars canon by George Lucas himself, prior to his redefining this canon with the advent of his Clone Wars cartoon, blasters and light sabers would short out when they came into contact with bodies of water. Drops such as rain and splashing had no effect other than sizzling. (lightsabers and lasers burn hot, not just bright.) According the books, contact with the water created a sort of feedback loop that would short out the lightsabers and blasters. (if blasters were under water, that is.) That being said, I'm sure that there were blasters that could have been modified in some way to work under water. There were no instances, however, of lightsabers working underwater until the advent of that season of Clone Wars.

George Lucas changed many things for this cartoon that were already considered canon, through books that were authorised by him. My conclusion is that he simply didn't read any of the material that he called canon, and didn't listen to his fans when they revealed that these things contradicted each other.

One solid example of lightsabers shorting out can be found throughout the Jedi Apprentice series.

  • The Clone Wars TV series was signed off on by Lucas for approval to be created, but he had no active part in the creation of any of the stories or episodes. He is only an executive producer. The show is considered T-Canon, and so the movies/novelizations of them still supersede anything from the Clone Wars show. – phantom42 May 27 '13 at 21:19
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Laser can go in water medium as it can go in air medium. The one and only condition is that the given medium must not absorb the laser light. If a medium is not absorbing laser light its temperature will not increase or it will not boil.

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    bogus, and self contradictory. – jwenting Jul 18 '13 at 10:05
  • It's not laser, it's plasma. – Overmind Jun 27 '16 at 12:21

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