Why is my laser’s effectiveness dependent on the target material?

Question

This is for a moderately hard sci-if (as in, as real as I can get it, minus FTL & humanoid aliens)

Side B has been captured by Side A, in order to get them to fight Side C. But Side B has a plan, they plan to use the lasers to only be effective on space dust, and not the enemy hull plating. This way, they can fool Side A, and at the same time, let Side C know they don’t actually want to fight, just put a show on.

Basically, I want my lasers to only be effective against certain targets at certain times. If a different type of target (e.g. armour plating material changes from titanium to steel) appears, they have to adjust the frequency (does that work for EM spectrum?) of the laser.

How do I make this work? Is there already some sort of research towards my “Spectrum Lasers” or is this going to some sort of hand-wave with a lot of flashing lights?

Side note: I know the tag says Sci-Fi, but the more realistic this can get, the better

Answer 1

Lasers do have different efficacy on different materials. The thing is that it is not very mysterious and will be quite visually obvious. Therefore probably not very deceiving.

The main optico-electronic property of the material that will decide how much it will interact with a laser of a given frequency and intensity is the absorption coefficient. If your absorption coefficient is very low with respect to a given radiation, it will not affect you a whole lot. If your absorption coefficient is high, you will absorb the incoming energy, for better or worse.

For more intuitive terms, let's focus on visible light to give some examples. An overall low absorption coefficient in the visible range would correspond to something that is either mostly transparent (like glass) or mostly reflective -or scattering- (like mirrors or bright white stuff). In both cases, they won't be very affected by a visible laser, either because it goes through or because it bounces off without heating the material much. Now, the absorption coefficient is a function of wavelength. That means that you can absorb more or less certain colors. This is essentially the reason why objects are colored. As an oversimplification, let's say that when you see an object as blue, it is because they don't absorb blue light (they drink up all other light, so that only blue is scattered off of them, and we see them as blue).

So basically, if you have a red laser and you shoot at a red target, it is going to be a lot less efficient than if you shoot at a black target. You can find plenty of youtube videos demonstrating that: https://www.youtube.com/watch?v=RAJKiu205yM This is the same reason why wearing a black shirt (which absorbs most visible light) under the sun feels a lot hotter than wearing a white shirt (which spits out a lot of light)

So a not-so-exciting solution is to tune the frequency of your laser to match the lowest point in the absorption spectrum of the target material. That will deal minimum damage, and this is tuned somewhat specifically to the material in question (I say somewhat because many different materials could share similar optical properties. It is not guaranteed that you spacedust does not have a similar minimum in the absorption spectrum)

However, if you're working with visible light lasers, it will essentially match the color of the ship. If the ship is black btw, it pretty much absorbs the whole visible spectrum so you won't be able to find a suitable frequency unless you use invisible lasers, either on the UV or IR side

Answer 2

The problem with weapons-grade lasers is that the sheer amount of energy they pump out means that even if the target is as close to perfectly reflective as practically possible against the frequency of light emitted by the laser, the relatively small amount of energy absorbed by the target is still going to be destructive.

However, lasers must be focused. A weapons-grade laser would likely have a large bore and an array of large lenses that can focus the laser beam on a relatively small area of the target without the energy densities involved destroying the lenses. By defocusing the laser against the target, the energy would be applied against a larger area, reducing the energy per area and thus reducing the heating effect that does damage to the target.

Of course, any observer would be able to tell that there had been a hit with minimal damage, given the reflected laser light and the relative lack of debris in the vicinity of the target.

Additionally, if less energy is pumped into the laser emitter, and/or the emitter's lasing substance (probably a gas) is reduced in quantity, the laser can be reduced in effective power.

Answer 3

Have the hull be some sort of super strong material.

The simple way is to have the hull just be really good. Maybe in the sci fi future, hull platings are much tougher because they need to deal with advanced weapons like lasers. They can then use a weaker laser that will destroy weak space dust but not tough hull plates.

Answer 4

Maybe lasers are not always homogenous: instead of true lasers (pure light), some some of the varieties of what we call "lasers" fire things like bursts of plasma, brightly-glowing physical particles, ion bursts, or even more exotic things like radiation rays and proton cannons (if you wanted to drift back towards the more hand-wavy side). Then you could have a perfectly reasonable excuse for why different ships have different durabilities: their shielding materials are better-tuned for a particular defense (no pun intended).

For example, Cortosis might be excellent at absorbing and redirecting high-energy particles and plasma, but the thin sheets will let solid particles (e.g. from a railgun) punch right through. On the other hand, Tritanium is a durable and sturdy material, impenetrable to physical projectiles, but it loses structural integrity when it absorbs too much energy from more exotic weapons. Figure out which the "enemy" ship is made out of, and use only the guns that their armour is designed to be strong against.

Other options:

• Could you take a step away from the material itself and use shields of some kind? A force field having a particular frequency seems plausible enough, and it would probably be pretty convincing to see the enemy ship's shields absorbing all the lasers (expected behaviour) instead of the lasers impacting and just doing no damage whatsoever (unexpected behaviour).
• The "force fields" could even be something overlapping with the hull: maybe the "force field" involves creating a particularly strong electromagnetic field along the hull, which deflects the plasma from the lasers. It wouldn't be immediately apparent what the field is keyed to, but logically there could only be one such shield on any given location of the ship, so you couldn't layer them. You could, however, potentially change it on the fly, "tuning" your shields to better match the damage profile of the enemy weapons.
• Could the lasers themselves be adjusted? Maybe the "attacking" ships has turned their weapons' power down significantly so it's basically just a fancy-looking flashlight or globs of light, instead of actually-destructive lasers.

Answer 5

Sabotage the targeting computer, not the laser

This is the space age meaning that between every laser and gunman is a powerful targeting computer and sensor system that works together to detect and identify a target, and then aim and calibrate the laser to hit it... because, no Civilization B gunman can just aim a laser at a target thousands if not millions of km away and expect to hit anything. Gunmen are just there to designate targets, and press the big red "fire" button.

So, you simply program the targeting computer to improperly focus the laser when a target matches a Civilization C signature. An asteroid, a comet, or even a friendly ship it works perfectly on, but against an enemy ship, the computer mis-focuses the beam causing it to spread out and into a 20m wide flashlight instead of a focused point of energy. To Civilization A's annalists, they will see the right amount of reflected light to confirm a hit, but unless they can get really close to the enemy ships to take a picture of what is going on, they will have a really hard time explaining why the lasers aren't doing any damage.