When a strong laser beam hits a mirror or lens, the glass always absorbs some of the light. This absorbed light quickly turns into heat inside the solid material. If the light energy arrives faster than the heat can spread out, the spot gets extremely hot.
Reaching the damage threshold means the material suffers permanent and visible harm. The surface might melt into a tiny pit or crack from the sudden physical stress. The exact breaking point depends heavily on the specific color of the laser light.
Some materials block blue light perfectly but melt instantly under an infrared laser. The timing of the laser pulse also changes the safety threshold completely. A laser that fires its energy in one billionth of a second causes massive shockwaves.
A steady laser beam gives the hot glass much more time to cool down safely. Surface quality plays a huge role in surviving a laser blast. A tiny scratch or speck of dust traps the heat and lowers the threshold.
A common misconception is that a perfect mirror reflects all light without taking damage. In reality, even the absolute best mirrors absorb a tiny fraction of the laser energy. This tiny absorbed fraction can easily destroy the mirror if the laser is strong enough. Engineers must know these exact limits when building strong lasers for cutting metal or performing eye surgery.