How Does a Laser Work?

LASER stands for Light Amplification by Stimulated Emission of Radiation. In a laser, energy (from electricity, flash lamps, or another laser) excites atoms or molecules in a gain medium (such as a ruby crystal, helium-neon gas, or semiconductor). When an excited atom is struck by a photon matching its energy level, it releases a second identical photon travelling in the same direction and phase — this is stimulated emission, first described by Einstein in 1917. Mirrors at each end of the gain medium bounce these photons back and forth, amplifying the light with each pass. One mirror is partially transparent, allowing a fraction of the coherent light to escape as the laser beam.