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So, the characters in my story are trapped in an abandoned city in a glass dome at the bottom of the ocean that has suffered damage from some sort of bombardment. I'm currently trying to work out the lore and history of that city (and the oceanic kingdom it is a part of), and, in specific, how exactly that damage occurred. I know that it resulted from a civil war between the denizens of the underwater cities and the denizens of the cities floating on the ocean's surface, but I haven't decided exactly what weapons the surface dweller used.

Now, obviously, however the conflict played out, the dome remained intact, so the surface dwellers must either have launched their attacks from within the dome, or had weapons that could pass through the dome. It's a world with magic in it, so I could just come up with some sort of magical weapon that fits the parameters I want if I have to, but I did get to wondering if lasers, being light-based weapons, could pass through glass without harming it. If so, could that, potentially, be applied to sci-fi laser cannons capable of creating the kind of destruction I'm describing? Would it be believable?

I did a little bit of googling on the topic, but the answers I was finding were more confusing than helpful. I often have a hard time grasping concepts unless they are directly explained to me, so if anyone has any insight on the matter, I would be very appreciative.

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    $\begingroup$ When firing your laser in both glass and water, be cautious of refraction angles ➡️⤴️! $\endgroup$
    – Tortliena
    Jun 28 at 19:42
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    $\begingroup$ I would think that the glass is a minor problem compared to the laser having to go through sea water. Sea water is a non-homogeneous, absorptive, dispersive medium. I wouldn't fancy the chances of the laser beam hitting anything much after going even a few tens of meters through sea water. $\endgroup$
    – AlexP
    Jun 28 at 19:46
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    $\begingroup$ @AlexP Lasers would still work fine when put against the dome so they only have glass and air to pass through. In fact, if lasers are otherwise known not to work well underwater this could give a nice element of surprise. $\endgroup$
    – JollyJoker
    Jun 29 at 7:46
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    $\begingroup$ One detail to remembers is that not only would a curved glass dome act as a lens to scatter lasers going outward, it would also focus lasers going inward. So, either the surface dwellers did not have laser-based WMDs, or the ocean dwellers lost the war. $\endgroup$ Jun 29 at 14:40
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    $\begingroup$ Doesn't every laser pass through glass by definition? What do you suppose the lens is made of? (On higher end lasers it might be sapphire or diamond or something, but your basic laser is going to have a glass lens.) $\endgroup$ Jun 29 at 18:56

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I have been working with lasers since my master thesis: laser can pass through glass without being attenuated in an appreciable way, mostly depending on its wavelength.

For example when I worked with a Nd:YAG laser, the base frequency in the infrared and the doubled frequency in the green could easily pass through the borosilicate glass I used as substrate, while if I aimed at using the UV part, I needed to switch to quartz substrates, which were more transparent.

The only thing you need to keep in mind is that the glass has to be clean and cooled. Clean because else any impurity will start absorbing the laser, heating up the glass and resulting in its breakage. In particular with weapon grade lasers. And cooled because, if it absorbs a 0.x % of the inpinging light, it will be a 0.x% of several kW, which will lead to heating up.

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    $\begingroup$ The only thing they need to keep in mind is that the glass has to be clean . . . and thin. (As any photographer will know, several centimeters of glass will easily absorb a significant fraction of the light; at the power of weapons-grade laser, those few percent will have an interesting effect.) The only two things they need to keep in mind is that the glass has to be clean and thin . . . and with parallel surfaces. (Or else it will act as a lens.) A glass dome at the bottom of the ocean will most likely be quite thick, curved, and not made of optical-quality glass. $\endgroup$
    – AlexP
    Jun 28 at 19:44
  • $\begingroup$ Thank you both for the feedback! It's sounding like it wouldn't be a very feasible method of attack since the conditions for it to work would have to be so perfect that they would seem contrived. $\endgroup$
    – Rurudo
    Jun 28 at 20:04
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    $\begingroup$ We are talking about lasers capable of destroying a city. In standard glass, if even a tiny fraction of the energy is absorbed it will instantly obliterate, I would think. A little impurity would heat up fantastically, causing melting which would increase the size of the absorbing area. Repeat until destruction. If the world has magic, perhaps a spell cast by the bad guys that makes the dome 100% transparent to laser light. That would work. $\endgroup$
    – Dan Hanson
    Jun 29 at 2:28
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    $\begingroup$ in other words: The fingerprints left by the guy leaning against the window result in the window starting to melt there. The melting window becomes opaque to the laser and melts the building. Also, melting glass is crazy hot - it's almost worse than metals if it wasn't so viscous. $\endgroup$
    – Trish
    Jun 29 at 13:24
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    $\begingroup$ A good answer to the question, but naïve. Firstly you talk about using borosilicate glass or quartz, would these be reasonable choices for an underwater dome? Can lasers be made to penetrate any kind of glass? You also talk about the glass being clean, remember this is an underwater dome, would marine growth affect the situation? How about 50 meters of water? Is there a frequency that can pass through all this water, marine growth, and the glass? Do we need a protocol of using lower powered lasers to slowly clear marine growth and clean the glass ready for bombardment? $\endgroup$
    – user91320
    Jun 30 at 0:38
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Yes, in principle. Although the glass in the dome will act as a lens, making it harder to focus the laser light. Also lasers are also not that great a weapon of mass destruction. If you have enough energy to do damage to something on the other side of the dome you may heat or weaken the glass. Rather than thinking about a beam of constant diameter, you could have the beam wider at the glass, and focused to a spot where you want the damage to occur. This would be hard to do in practice, but if you knew the details of the dome etc, it could be figured out.

Some other things that might not matter to your story but might be something to think about:

Lasers light propagating underwater don't tend to work very well. Seawater is very absorbing in the yellow orange, red and UV and infrared and even in the blue green window is more absorbing than air. You also have particulates that scatter the light, and any blue or green laser beam would be very visible. Even with sunlight, it gets dark pretty fast underwater, by the time you are about 200 meters underwater almost all the sunlight (even the blue green) is absorbed. The strong absorption means that probably the lasers need to get pretty close to do a lot of damage. Too much power you cold also start heating up the water in the beam and that could cause bubbles or reflections reducing the amount of power that can reach your target.

Going from the higher index of refraction water or glass to the air inside the dome you could have total internal reflection. The effect of that is that 100% of the laser beam would reflect away from the dome unless the laser beam within a range of angles called the critical angle. So the laser beam to work the best would need to be perpendicular to the glass surface or at least within the cone defined by the critical angle.

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  • $\begingroup$ Awesome, thanks for the detailed comment! I'll definitely have to keep all that in mind as I think this over. May be more trouble than it's worth trying to explain the damage with lasers, so I'll probably just come up with something different, or write the battle as having taken place within the dome. $\endgroup$
    – Rurudo
    Jun 28 at 20:01
  • $\begingroup$ The glass would either act as a very weak lens (meniscus lens) if formed in one piece, or if made up of flat plates (as in a geodesic dome) it won;t act as a lens at all. $\endgroup$
    – Chris H
    Jun 29 at 15:39
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I'm a +30 yr veteran with lasers. The glass is not the problem... the water is.

Ocean water absorbs a lot of light. Laser communications struggles with submarine environments (see: https://www.sbir.gov/content/unique-high-powered-bluegreen-laser-communication-system-submarines-0 and https://archive.navalsubleague.org/1988/laser-communications-with-submarines).

Given the exotic wavelengths required to get small signals (laser communications) through ocean water, I would consider it completely impractical to user laser weapons in the ocean. In additional to materials that absorb, there are many particles in sea water that will scatter visible light.

I would consider re-writing your story to imply that an undersea dome is well protected from laser / directed energy weapons.

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    $\begingroup$ Agreed; the various plankton and other particles in the water are going to do a lot of work reducing the power of this laser. $\endgroup$ Jun 29 at 19:13
  • $\begingroup$ Truth be told: YHVH simply doesn't like to do all the work calculating light through water. $\endgroup$
    – theDoctor
    Jun 29 at 19:39
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    $\begingroup$ Makes me think of Stargate Atlantis where the Ancients sunk their city under the ocean for protection against the Wraith energy weapons. $\endgroup$
    – Nosajimiki
    Jun 29 at 20:02
  • $\begingroup$ What if... energy absorption by the water is intended, with the goal to create waves or wave patterns that harm the cities of the surface-dwelling people? You would need much energy, and of course most of the energy would be delivered right above the dome, but you can use multiple lasers to distribute the load and locate the energy maximum (not much) higher above the dome. Additionally, could correction optics reduce the optical influence of the domes glass? $\endgroup$
    – syck
    Jul 2 at 9:00
  • $\begingroup$ Not sure what you mean... please describe... The energy absorption in the water is very distributed. It takes collimated or focused laser light and makes it diffuse; like shining a flashlight in the fog. The fog will light up, but the energy is so widespread, no damage can occur. $\endgroup$ Jul 4 at 8:08
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You can't use lasers or guns under water

Lasers: Water refracts and absorbed lasers. Every 10m that your laser travels, it will loose 1/2 of its potential energy. So even if your "bottom of the ocean" is somewhere relatively shallow like on a continental shelf at a depth of 100 meters, that means your laser will lose 99.9% of its energy before even making it to the dome. Furthermore, as it heats up the surface water boiling it, the bubbles and steam will cause extra refraction making the water essentially opaque to your laser long before your beam can cause any significant heating to the dome.

Bullets: Drag is amplified based on how fast you are going and your proportional size. Because water already has a lot of drag, and bullets do all of their acceleration on the front end, a standard firearm can only pernitrate a few feet of water before coming to a complete stop and just sinking at it's terminal fall velocity which will not be very fast at all. The most specialized underwater bullet in the world, the CAV-X, has an anti-material range of 17m and an absolute maximum range of 60m which still is not enough to reach your city.

This leaves you with Torpedoes and Depth Charges

When going for range under water, big, slow and steady wins the race. A torpedo can go much farther than a bullet using the same amount of energy. Part of this is because drag is exponentially proportional to your speed. While an average bullet travels at over 300mps, an average modern torpedo moves at only 25 mps, using sustained acceleration. This means the an average torpedo has ~150 times less drag for its surface area than your average handgun. Also, keeping the propellant onboard means that the torpedo tends to be much bigger and longer for its proportional effect compared to a bullet. Because the square-cube law applies to resistance, a large object moving though water tends to experience much less resistance compared to its size than a bullet. It's like the difference between a bug being able to stand on water and a human sinking into it. Torpedoes can also use more energy dense fuel sources than bullets, and can take advantage of creating a sustained bubble jacket around them for more thrust and reduced drag. All these advantages add up to ranges that are measured in kilometers, not meters.

If you're going for a more low tech weapon, then use depth charges. Powered by gravity, you can simply sink them and have them explode when they hit the dome.

Why the underwater city did not completely fail from the bombardment

In the contest between a large explosive weapon and a glass dome, it sounds safe to say that the explosive will win hands down. That said, when building for large scale habitat for hazardous environments, redundancy is a key safety feature. Many large naval ships and submarines use double and segmented hulls allowing them to take structural damage without losing the whole ship. So, it is safe to say that your under water people building something the size of a city would do the same.

The way a double hulled dome would work is that you have 2 domes, one inside the other. The torpedoes and depth charges could blast holes in the outer dome, but unless two shots hit in the exact same place, the inner dome would be fine. It is also important to make sure the dome is cellular because glass shatters; so, if the outer hull is made up of 1 meter glass hexagons, then when a torpedo hits one, it would limit the damage to a smaller area. Basically, your outer dome would work like the ceramic tiles in often found in flexible riffle armor where they are designed to shatter.

As for segmenting your city, you are probably picturing a giant dome covering a bunch of terrestrial style buildings. But this is horribly unsafe meaning you are just one mechanical failure away from instant death. Instead each building under the dome could be an individually functional underwater environment. This way, if the dome ever breaches, they can all just retreat indoors while the engineers patch and drain the dome. Or, and ever better approach would be to ditch the giant dome all together and go with many smaller domes connected by airlocks.

The best thing about segmentation is that you can plausibly lose specific systems an not others. So, perhaps the city still has power and O2 scrubbers so your heroes don't instantly die, but perhaps the water purification dome and communications systems were destroyed during the bombardment because they were in domes that got hit.

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I had laser eye surgery a long time ago and they have an interesting approach to destroying minute amounts of material behind a lens (aka my eye). They use two half strength beams that when focused independently intersect at the spot (delivering 100% power) where they want to burn away lens material. This was done repeatedly to remove a thin layer over a wider surface area with a high degree of 3 dimensional precision and minimal collateral damage.

If you multiplied this several times and had 10 or 100 or more lower strength lasers targeting specific high value targets, each laser would interact minimally with the barrier/glass dome reducing the effects of dirt/defects/heating. If the dome was underwater, sufficient ambient cooling might occur. All you need is multiple attack vectors with direct line of sight to the target (easy when attacking from above in a world with gravity sufficient to hold down water).

Another way to look at this is how a natural concave lens works or even a magnifying glass. It bounces/refracts light off a dome shaped surface/lens to focus on one point in the middle. In effect it's an infinite number of "lasers" pointing low power light into one spot of high intensity power.

Don't quote me, but I believe this multi laser approach might also be in use for military grade anti missile defense "shields" since it naturally provides layers of redundancy and reduces the risk of friendly fire/innocent casualties. For example slicing the wings off a nearby plane in the line of fire as the laser is tracking the target. Note that in this situation, the laser is aided by the explosive nature of the missile and fragility of precision guidance systems. It's doesn't have to produce devastating destructive power on it's own.

Edit: there could be quite a cinematic opportunity here. It would literally be raining down lasers. You'd have people see little red dots appear like rain drops unaware of what's happening. A side on shot would show a "downpour" of red lines, and everything bathed in red. The lines would then swing around searching for a target. Masses of them would swarm on people or anything combustible. Then the explosions would start; barrels, gas mains, aircraft wings in mid flight... panic and people running seeking shelter. Large clouds of smoke and debris would refract the lasers into a blinding lightshow. When the dust clears, slowly the lasers switch off one by one like the lights on a stage.

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  • $\begingroup$ I was going to write something like this. Many hundreds of lasers from many directions converging to start fires inside the dome(s). $\endgroup$ Jul 1 at 11:14
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Existing answers show that lasers would kind of work. But I don't think they would work all that well. If you went with lasers you should use pulsed lasers, their damage is pretty "mechanical" and instantaneous. The obvious solution here is that laser was used to light up something explosive in the city, next paragraph shows why direct laser damage isn't too good here.

1kJ pulse creates hole in water of about 1 cm in radius. Good enough vs people, but a great many shots would be needed to take down a building. However, damage threshold of glass is a bit of a problem. Add nasty stuff on the dome surface, and you would be very lucky if you get damage threshold at 10J/cm2, but more likely only 1J/cm2. So, your blaster of kJ laser pulses would need a "window" with diameter of say 0.2m... and it is just a person-killing laser that wouldn't do much structural damage.

But, there is a better solution.

Sound/shock waves. Given water setting sonar and the like are very well developed as should be various explosives. So, how the destruction went? Someone created many shock waves around the dome, propagating towards the dome. These shockwaves interfered constructively in the center, producing huge destruction (direct damage or resonance). The dome probably shook quite a lot too but managed to withstand this - after all, it is supposed to protect you from explosions to some degree, isn't it? How were shockwaves created? Using tons of computer controlled depth charges which produce little explosion making a strong mostly directional shockwave. You can use lasers for that, but ordinary explosive works too. Previously, such charges might have been used for imaging/scanning/... (think spy drone sending pings and listening for echo), and nobody was able to use them offensively in this way before - no good enough timing. Then someone managed to connect millions with computer control and a simulation of propagation, making the big kaboom.

A typical dome shape is helpful because stuff you send against the dome and gets reflected is dissipating, while stuff that gets through is converging to the center. This is similar to the laser, though you generally wouldn't imagine to cover the whole dome with lasers. Besides, unique advantage of sound is that even reflections from the ground that reflect from the dome once again converge to the center, amplifying damage. So, you can detonate some charges now, and when the wave reflected from the ground reaches dome, send another shockwave to add to the first one. Then another, and another - essentially pumping energy inside the dome, until everything inside is destroyed. Another nice thing I see is that this approach can use resonance for mostly no damage to anything except the primary target, if its resonance modes are known.

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The glass forms a thin layer upon being struck by an explosive.

The glass dome is self sealing and repairing, and upon being damaged forms a bubble on the inside which tends to deflect explosives and other debris from penetrating.

This glass is however quite thin, and a laser weapon close by can blast the city from the outside after a collision. The surface dwellers would drop depth charges, and then have close by submarines rush in while the dome was repairing and blast those inside with laser blasts.

It isn't plausible that a dome of glass would be fine if it couldn't self repair, with lasers or other weapons.

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If I wanted to destroy an underwater dome, I'd use a depth charge. It slowly sinks through water (optionally camouflaged, so the underwater guys won't know that it is dangerous), and explodes near the dome, rupturing it.

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Siege

There's no practical way to damage an underwater structure with a laser. There's just too much water and marine life in the way. Especially if it's beneath a glass dome and you don't want to destroy the dome.

There are simpler things to do. First off would be to blockade the underwater city. Everyone needs food to survive, is the underwater city totally self reliant? Can it be cut off from its supply chains? Can we hinder food growth within the city - eg deploying plastic balls to cover the ocean over the city and block sun?

Simply preventing the city from getting food is simple, cost effective, and safe. This will leave the city in tact but decaying.

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A siege

A city enclosed in a glass bubble with few doors is the perfect target for a siege. Rather than bombing the city you might say that is was isolated for some time until most of the inhabitants starved to death.

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