Laser- (as per wikipedia) - a device that generates an intense beam of coherent monochromatic light (or other electromagnetic radiation) by stimulated emission of photons from excited atoms or molecules.
When I was a graduate student I had the opportunity to learn about several ways in which lasers are generated. The basic idea is that once you can keep the electrons cascading up and down between certain energy states, you can then "pump" them up and down and it generates light in a very reliable way. These days, we use lasers for many things: eye surgery, etching, sights on guns, to detect speeds, to measure whether something is straight, the list goes on and on. Back in the last century (can't believe I'm writing that), it was widely thought that lasers were only red, because this was the only type of photons that could be reliably pumped. However, nowadays, we are able to produce lasers which are red, green, blue, blue-violet, and purple.
