Laser, a device that generates or amplifies waves of visible light or of such other forms of electromagnetic energy as ultraviolet and infrared radiation. The word laser is derived from the principle of the device itself: light amplification by stimulated emission of radiation. A related device, the maser, is used to generate or amplify microwaves.
A laser can produce a beam of coherent, highly directional, and highly intense light. The light is coherentthat is, the waves making up the beam have the same wavelength and are in phase with each other. Light from ordinary light sources does not have this property. The laser's light is directionalthat is, it does not spread out over a large area after it leaves its sourcebecause the light is coherent and is emitted from the laser in a single direction. (By contrast, light from an ordinary lamp bulb spreads out in all directions.) Because it does not spread out and because its waves are in phase, the light is also intense, diminishing little in brightness as it leaves the laser.
Most types of lasers can produce light of only one or a few specific wavelengths. Some types, however, can be adjusted to produce light at any of a wide range of wavelengths. Some lasers, called continuous wave lasers, produce a steady beam of light. Others, called pulsed lasers, produce light in pulses, each lasting only a fraction of a second.
