text: skyman,\n\nthis is kind of a big question since there are very very many different kinds of lasers that work very differently.  i'll try to answer it in a general way.\n\nLASER stands for Light Amplification by Stimulated Emission of Radiation.  i'll get to exactly what that means in a minute.\n\nthe earliest laser was a rod of ruby crystal, with mirrors on both ends with the shiny faces pointing towards the crystal, and some very bright flashbulbs around the sides.  one of the mirrors was a perfect mirror - 100% of the light was reflected - and the other was not quite - 99% or so was reflected but about 1% passed through.\n\nthe way it worked was that you'd fire the flashbulbs, which would shine very brightly into the ruby crystal.  all the atoms in the ruby absorb some energy from the light and their electrons go to a higher-energy state - this is called a "population inversion" and is important to the operation of a laser.\n\nanyway, before long some of those atoms will start emitting photons and settling back down into their ground state.  some photons will be going the right direction so that they bounce back and forth between the mirrored ends of the rod, over and over.  \n\nevery time a photon goes past an excited atom, it causes the atom to radiate a photon too, in the same direction and in phase with the original photon.  this is the "stimulated emission" in LASER.  before long this cascades into a torrent of photons, all bouncing back and forth between the mirrors - about 1% of them leak out and you get a laser beam.\n\nthere are a lot of other designs for lasers, but they all work on the same fundamental principle.  you have some kind of material that can emit photons when it's in an excited state, and you put it between two mirrors in what is called an optical resonator or optical cavity.  of course there are a lot more technical details that i'm skipping over, but hopefully you get the basic idea.