A laser is a device which is built on the principles of quantum mechanics to create a beam of light where all of the photons are in a coherent state - usually with the same frequency and phase. (Most light sources emit incoherent light, where the phase varies randomly.) Among the other effects, this means that the light from a laser is often tightly focused and does not diverge much, resulting in the traditional laser beam.
How a Laser Works
In simplest terms, a laser uses light to stimulate the electrons in a "gain medium" into an excited state (called optical pumping). When the electrons collapse into the lower-energy unexcited state, they emit photons. These photons pass between two mirrors, so there are more and more photons exciting the gain medium, "amplifying" the intensity of the beam. A narrow hole in one of the mirrors allows a small amount of the light to escape (i.e. the laser beam itself).
Who Developed the Laser
This process is based on work by Albert Einstein in 1917 and many others. Physicists Charles H. Townes, Nicolay Basov, and Aleksandr Prokhorov received the 1964 Nobel Prize in Physics for their development of the earliest laser prototypes. Alfred Kastler received the 1966 Nobel Prize in Physics for his 1950 description of optical pumping. On May 16, 1960, Theodore Maiman demonstrated the first working laser.
Other Types of Laser
The "light" of a laser does not need to be in the visible spectrum but can be any sort of electromagnetic radiation. A maser, for example, is a type of laser that emits microwave radiation instead of visible light. (The maser was actually developed before the more general laser. For a while, the visible laser was actually called an optical maser, but that usage has fallen well out of common usage.) Similar methods have been used to create devices, such as an "atomic laser," which emit other types of particles in coherent states.
There is also a verb form of laser, "to lase," which means "to produce laser light" or "to apply laser light to."
Also Known As: Light Amplification by Stimulated Emission of Radiation, maser, optical maser