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My character can burn calories and turn it into a beam of light a few centimeters in diameter with nearly 100% efficiency. What wavelength would be best as an antipersonel ability, calorie for calorie? The range will be under 100 yards, and some enemies will be wearing full plate armor. He can adjust the output, but would burning around 1000 calories in one shot be enough to kill one enemy with a headshot?

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    $\begingroup$ I hope this person always has literally instant meals on hand because forcing a handful of shots would probably weaken them to the point of starvation. Probably best to just carry a gun, then a knife to ensure you don't waste 1000 calories on a missed shot in close quarters and starve to death. $\endgroup$ – DKNguyen Oct 26 '20 at 16:08
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    $\begingroup$ ? Energy is energy and power is power. 1000 kcal (I supposed that by "calories" you mean food calories, calories-kilogram) over 1 millisecond is one thing, 1000 kcal over the one whole day is quite another. In general, humans are poor reflectors, so that you can assume that about 3/4 of the energy will be absorded by the target area. $\endgroup$ – AlexP Oct 26 '20 at 16:10
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    $\begingroup$ 1000 calories is about 6 megaJoules. Over 100ms, that's 60 megaWatts. The US Navy has a 50kW laser so it should do some damage, assuming a 100ms burst is long enough, which it might not be. But still lots of headroom to extend the time and still remain above 50kW. It would probably be easiest just to blind people though, even if you have to go past protective eyewear and then go in with normal arms after...much lower power and energy required $\endgroup$ – DKNguyen Oct 26 '20 at 16:17
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    $\begingroup$ 1000 food calories is 4.184 mega Joules, not 6. But if you can transfer the energy to a vital organ you are quite likely to cook it. 1 food calorie raises 1 kg of water by 1 C. So if you could get 3/4 the energy into, say, 2 kg of brain, you would be quite likely to raise the temperature to boiling. $\endgroup$ – puppetsock Oct 27 '20 at 0:00
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    $\begingroup$ You might want to narrow the focus of the 'beam'. Instead of putting all the energy into a beam 'a few centimeters in diameter' you practice until you can get it down to less than a centimeter or narrower. By doing so you concentrate any given time span of output into a much smaller volume, greatly increasing the amount of watts per square millimeter of surface area striking the target. Also learning to 'change frequencies' might be useful. You then have the option of dazzling/blinding an opponent with visible light, starting small fires etc with infrared or toasting a target with microwaves. $\endgroup$ – Mon Oct 28 '20 at 2:30
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Probably microwave 1 to 10 cm

There's a reason we use this to cook food, water is a good receiver of energy at this wavelength, and humans are mostly water. So if you want to cook someone's brain, that might be your best bet. It has the range you want too, microwave communications span 1 km or so, but it'll be less for a high energy weapon. The key is that you have to do it in a burst, not a sustained ray.

FYI, yes I know that technically the classification of microwaves is larger than this, but this is the range that is most commonly used in microwave ovens.

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I agree with Mathaddict for people naked or wearing regular clothes, however the metallic armor might reflect some of the MW radiation.

Use gamma rays, those will go better through armor. It also adds radiation poisoning, which is everlasting - so you know that if the bugger survives they're less likely to be alive or in shape to go for revenge.

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  • $\begingroup$ The problem with gamma rays is that either an extremely short burst is necessary, or several hundred thousand calories are necessary. $\endgroup$ – Salami-tsunami Oct 26 '20 at 19:07
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    $\begingroup$ You're right about the reflective properties, but this is more a light problem than a frequency problem. That said, microwaves can still cause hot spots and arcing in corners within a metal structure, though probably non lethal. $\endgroup$ – Mathaddict Oct 26 '20 at 22:22
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By definition, 1 calorie is (roughly) the energy needed to warm 1 gram of water by 1°C

So, if the 1000 cal are focused on 1g of water, it will reach 1000°c and simply explode. If it were spread over 100g, it would raise temperature by 10°C and could cause minor burns

BUT as @AlexP explains, calorie ≠ Calorie == kilocalorie == calorie (in nutritional terms)

Assuming you are talking about food calories, that is a million real calories and that's enough to raise the temperature of 1kg of water by 1000°C. The vapor explosion might be similar to a hand grenade.

About the wave length, I'd suggest you use infra-red or visible light. Those wave-lengths are immediately absorbed by the skin so the whole energy of your beam will be absorbed by a very low quantity of matter (that's the scenario where you heat only 1g). This will cause a vapor explosion with the maximum detonation velocity localized DIRECTLY on the skin, sending a terrible shockwave inside the body.

When it comes to armor, it would totally depend on what kind of armor. Is it medieval metallic armor? A ceramic modern thing? Was it designed to protect the wearer from your magical ray of death?

Basically, I'd say that medieval plate armor (much padding with a layer of shiny steel) would offer good protection.

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