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In a futuristic setting where people are worried about enemy spacecraft doing things like dropping bombs from orbit, and are building ground-based lasers to shoot back,

Given that such weapons have the disadvantage of having to shoot up through atmosphere (putting them on high ground ~2 km helps a bit, but only a bit),

What is the optimum wavelength to minimize absorption and scattering by air (and preferably by clouds)? It's definitely going to be some way into the infrared, but what exact wavelength would be best?

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  • $\begingroup$ Maybe you will find this interesting gsp.humboldt.edu/olm_2016/Courses/GSP_216_Online/lesson2-1/… . Otherwise there are several wikipedia articles about such a topic. I think as it stands this question doesn't really display a lot of research effort. It's also not really about world building $\endgroup$ – Raditz_35 Apr 18 '18 at 11:15
  • $\begingroup$ you can see the Sun outside. that's should give you a huge hint... $\endgroup$ – L.Dutch Apr 18 '18 at 11:35
  • $\begingroup$ Have you read about military laser projects from the past? On some, wavelength was not classified. $\endgroup$ – Mołot Apr 18 '18 at 11:37
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You want wavelengths which are minimally absorbed and minimally scattered.

http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1/atmosphere.html absorbtion

Another issue with your laser is scatter. The shorter the wavelength, the more the scatter. So: even though by this graph it looks as though blue and near ultraviolet are not absorbed they are scattered. Surprisingly to me the far infrared is well absorbed. So probably you would want visible light in the greens or yellows.

Or if you wanted to get all scifi, long but not really long wavelength rays to get into that super low absorbance radio wave band. I am not sure how energetic radiowaves would drop off their energy in your target, though. They might pass right through that too.

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  • $\begingroup$ I am thinking of those scenes in Expanse where the railgun bullets go thru the cabin spaces and the crew keeps doing what they are doing. In this scenario I envision a defensive mode where the ship becomes transparent: one sees the crew standing and walking around in empty space. They can see the lasers flashing thru the transparent hull. $\endgroup$ – Willk Apr 18 '18 at 15:53
  • $\begingroup$ What about that band around $10 \mu m$? Sure, absorption seems to be about 15%, but on the upside you probably have way less scattering. Is there any numerical estimate on how much scattering you get? $\endgroup$ – Alice Jul 4 '18 at 8:40
  • $\begingroup$ @Alice - You are right about that long-wave infrared frequency. It might be good for the laser although less awesome because invisible. Rayleigh scattering is what scatters the blue frequencies and Mie scattering the long wave. Mie scattering depends a lot on size and density of particulates in the air. Calculate Mie scattering for your wavelength and particulate size here! omlc.org/calc/mie_calc.html $\endgroup$ – Willk Jul 4 '18 at 18:38

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