OT: Upper limit on Lasers, range and power....

[Will]

Well, a laser is just light. It is, in fact, an acronym for Light Amplified by Stimulated Emission of Radiation. Basically, you apply some energy to some kind of material, and you get it to emit light in a specific direction. Normally, light will move out in a radial pattern, but lasers move in a very close approximation to linear. You could aim a laser at some point, and the radius of the dot would increase by something extremely slight like 1cm/light-year (the exact amount would depend on the kind of laser, and any refraction along the way).

The power of a laser is kind of like the power of using a magnifying glass to focus sunlight, only without the need to focus. If a laser hits a mirror, what happens depends on the percentage of light that the mirror reflects; there is no perfect mirror, there is always at least a little absorption or scattering or transmitting. If the mirror reflects 99% of the light energy, and absorbs 1%, and it takes X joules to vaporize the mirror material, then the laser would need to be 100*X joules to not be reflected. And, of course, materials have different absorption/reflection/transmission/scattering properties based on frequency. So you could have armor that would handle Y joules of energy from a green laser, but would vaporize with the same energy microwave laser.