Laser, pronounced LAY zuhr, is a device that produces a very narrow, powerful beam of light. Some beams are thin enough to drill 200 holes on a spot as tiny as the head of a pin. The ability to focus laser light so precisely makes it extremely powerful. For example, some beams can pierce a diamond, the hardest natural substance. Others can trigger a small nuclear reaction. A laser beam also can be transmitted over long distances with no loss of power. Some beams have reached the moon. The special qualities of laser light make it ideal for a variety of applications. Some types of lasers, for example, are used to play music, read price codes, cut and weld metal, and transmit information. Lasers can also guide a missile to a target, repair damaged eyes, and produce spectacular displays of light. Still other lasers are used to align walls and ceilings in a building or to print documents. Some lasers even can detect the slightest movement of a continent. Lasers vary greatly in size. One is almost as long as a football field. Another type is as small as a grain of salt. A typical laser has three main parts. These parts are (1) an energy source, (2) a substance called an active medium, and (3) a structure enclosing the active medium known as an optical cavity. The energy source supplies an electric current, light, or other form of energy. The atoms of the active medium can absorb the energy, store it for a while, and release the energy as light. Some of this light triggers other atoms to release their energy. More light is added to the triggering light. Mirrors at the ends of the optical cavity reflect the light back into the active medium. The reflected light causes more atoms to give off light. The light grows stronger, and part of it emerges from the laser as a narrow beam. Some beams are visible. Others consist of invisible forms of radiation. There are four main kinds of lasers. They are solidstate lasers, sem...
