Well in a real physics universe, armor would be good for nothing against attacks by mass driver type weapons, but could be useful against non-penetrating nuclear weapons, light based weapons.
A projectile accelerated (in reference to it's target) fast enough (let's say a 1000 km/s) would have so much energy that it would probably vaporize every kind of armor in it's way.
E,kin=½*m*v²
Let's say, 10kg of mass, with a speed of 1000km/s
½*10*1'000'000²=5*10^12 joule, which equals about 1Kt of TNT (1Mt = 4.18*10^15 J) which is about one fifteenth of the yield of the Hiroshima atomic bomb.
A projectile of 10kg mass would have (if it's spherical) a diameter of about (let's say it's made of Osmium or Rhenium (very high boiling point and density of about 22g/cm³) 10cm. (We could also use DU which isn't bad at a density of ca. 19g/cm³.)
So, we have the energy of a small tactical nuke (1 Kt) inside a small ball (10cm diameter) with a velocity of 1000km/s impacting on an armor (some sort of Wolfram/ceramic maybe?) Wolfram has a Lv of 43.5*10^5 J/kg.
m=Q/Lv (with m being mass which will boil, Q the amount of energy) we get:
M=5*10^12J/(43.5*10^5J/kg)=1.15*10^6kg ~ 1'150t.
So one shot of this weapon would be able to vaporize about 1'000 tons of armor on a ship.
I dunno about that, but it doesn't really sound like a ship could withstand such sort of damage for very long.
And I don't think that reflective armor would work very well against it, since it's of such a small size and with such a high velocity, that I don't think it could be deflected.
Against lasers and non-penetrating nuclear weapons, armor could work quite well imho. It would probably be made of something with a high thermal conductivity (some sort of crystal perhaps?), which could be rotating as well, to dissipate the heat/energy as good as possible.
At least that's how I think armor would work. Sorry for the crude calculations above, but I'm too lazy to get better formulas out. 
