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Many nations are developing hi-energy laser weapon. My question is, what if target is coated with mirror like coating? Can laser (since laser is still light) penetrate mirror? If it can then how is it possible?

Regards

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1 Answer 1

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If you look at reflectances of common materials used to make mirrors with (for example, the topmost graph found on this wiki page), you'll see that not 100% of the light is reflected, especially at the shorter wavelengths.

I'm still looking for a somewhat better source for similar curves for household mirrors, but I know that the idea is roughly the same -- it's actually pretty difficult to make a mirror that reflects 100% of all incident light.

The light that does not get reflected gets either absorbed (mostly) or transmitted (usually only for very thin film mirrors). The portion that gets absorbed is transformed into heat, which is transferred into the mirror material.

Therefore, if you shine a short-wavelength, high-power laser beam directly onto a mirror with the intention of damaging the mirror, the power of the laser must be great enough to ensure that the portion of the light that gets absorbed is great enough to heat up the material sufficiently (and fast enough) to melt it. This makes most laser weapons only really effective on

  • surfaces with low reflectance (plastics, certain composites, human skin, etc.)
  • thin-walled structures (fuel tanks, etc.)
  • sensitive electronics (camera's, targeting systems, etc.)
  • etc.

Laser weapons (at least, currently) serve a different purpose than ballistic weapons; they are more a tool for precision work at large distance.

These are the same sorts of problems a laser cutting tool encounters. Most of these sorts of machines can cut effortlessly through plastics, wood, ect. with high precision. However, when cutting through metal, they can only cut through relatively thin sheets (1cm aluminum already presents too much of a challenge for most machines) because of the high reflectance of most metals. The efficiency of the machine is also not too great with sheet metal -- a 50 kW machine will normally transfer only a handful of watts of heat to the focal point.

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I would think that there can be 2 steps if the mirror is a coating. First produce enough heat to destroy or reduce the reflective properties of the coating, and then more heat can be absorbed to burn or melt whatever is behind. Of course, that is less effective with a metal sheet, but it is with coated composite material. Metal also has the defense of fast heat dissipation. –  babou Jul 11 '13 at 9:43
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A reflective coating also has the downside that it can get dirty, through dust particles in the air. I also remember seeing a video in which they mentioned that laser-cutters also have to deal with, namely that if smoke of the burned material can stick to the mirrors which guid the laser beam. –  fibonatic Feb 21 at 16:33
    
If I understand you correctly, increasing the intensity of the light would increase the heat transfer because a small percentage would be absorbed by the material. And increasing the frequency would likewise increase the percentage of photons not reflected by the mirror. –  kleineg May 6 at 16:47
    
@kleineg: sounds about right, somewhere there is some "worst" trade-off between intensity, frequency and energy usage. You'd have to take care not to increase the frequency too much, because many materials would become transparent to it (think X-ray). Point is, the most destructive combination of these parameters is really very material-specific; I suspect all-round laser weapons therefore use the true army style brute-force approach of just using ridiculously high intensities at a frequency that's most effective for the most common metals used on aircraft/ships...but I'm no expert :) –  Rody Oldenhuis May 7 at 7:52

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