Will high power laser penetrate mirror? Many nations are developing high energy laser weapons. My question is, what if target is coated with a mirror-like coating? Can a laser (since laser is still light) penetrate a mirror? If it can, then how is it possible?
 A: 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.
