Acronym for Light Amplification by Stimulated Emission of Radiation; a device that produces a powerful, highly directional, monochromatic, coherent beam of light. Laser consist of a transparent cylinder with a reflecting surface at one end and a partially reflecting surface at the other. Light waves are reflected back and forth, some of them emerging at the partially reflecting end. The light source may be a ruby, whose chromium atoms are excited by a flash lamp so that they emit pulses of highly coherent light, or a mixture of inert gases that produce a continuos beam, or a cube of treated gallium arsenide which emits infrared radiation when an electric current passes through it.