A laser produces a highly-concentrated beam of light, which can be in the visible or invisible spectrum of light.
When lasers were invented in 1960, some scientists referred to them as a 'solution looking for a problem'. In spite of those misgivings, lasers are today highly-specialized devices used in thousands of highly-varied applications in every section of modern society, such as pointing devices for presentations; computer mice; the reading / writing mechanism of CD and DVD players and computer storage devices; industrial cutting tools for steel and plastic; range-finders used in speed traps; medical devices such as the Excimer and JAG lasers; playstations; military applications such as range finders and night vision and many more.
Lasers come in many different forms, but the broad principles on which they work are similar.
A laser emits light through a process of 'optical amplification', based on the stimulated emission of photons. The word laser is an acronym for 'Light Amplification by Stimulated Emission of Radiation'. The term 'light' broadly denotes electromagnetic radiation of any frequency - not only visible light, but infrared, ultraviolet, X-rays and so on. The emitted laser beam is notable for its high degree of coherence, which means that the output beam is narrow, often referred to as a so-called 'pencil beam'.
Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can be launched into beams of very low divergence to concentrate their power at a large distance.
Temporal coherence implies a polarised wave at a single frequency, of which its phase is correlated over a relatively large distance along the beam - or a beam of light that has a very 'pure' colour or, as it is also known, 'monochromatic'.
Lasers range in size from microscopic diode lasers with numerous applications, to football-field sized neodymium glass lasers used for inertial confinement fusion, nuclear weapons research and other high-energy physics experiments.
Light of a specific wavelength, passing through the gain medium, is amplified.
For the gain medium to amplify light, it must be supplied with energy, a process called 'pumping'. The energy is typically supplied as an electrical current or as light of a different wavelength.
The most common type of laser uses feedback from an optical cavity - two mirrors on either end of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and is amplified during each pass. Typically one of the two mirrors (the output coupler) is partially transparent. Some of the light escapes through this mirror. Depending on the design of the cavity (whether the mirrors are flat or curved), the light coming out of the laser may spread out or form a narrow beam.