The incredible science of laser (thanks for correcting me) You're holding a lamp in your hands. The light arriving from the lamp illuminates the place by all the directions of the space, very useful when you're in the dark. Now, imagine you're also in the dark but you want to focus simply on a little stain. A real lamp isn't useful anymore and you need a source which illuminates just the stain. Since the beginning of the humanity , lot of scientist have worried about a solution for this problem. A discover by Einstein in 1917 led to the birth of a ray with a very small radius: a laser. Nothing very suprising until now, but do all of you know how a laser works ? First, as an information, "laser" comes from "Light Amplification by stimulated Emission of Radiation". One should Bear in mind that each atom has two possible states: a neutral state E1 and an excited state E2 (E1-When a photon penetrates a neutral atom (that is to say at a level of energy E1) the latter absorbs it and the atom becomes excited (at E2) -An excited atom can emit naturaly a photon and become neutral (1) -When a photon penetrates an excited atom, the latter emits two photons in exactly the same directions as the first one (2) In a laser there is a huge amount of neutral and excited atoms between two mirrors. Most of them are excited. When the first one emits naturaly a photon (1), it will engender by butterfly effect the emmision of most of the other photons from the other atoms (2) always in the same direction as the first one. Thanks to the mirrors, the accumulation of photons in the same direction creates the unidirectionnal laser. It's very funny when we think that we began with a little excited atom ! The differents colors of laser that we can see in the world simply come from the fact that the difference of energies (E2-E1) is proportionnal to the frecuency of photons emmision and so the wave lenght emitted hence the differents colors.