The discharge tube of an MCL-1500, an industrial gas laser used to cut metal. The discharge tube, seen here containing a purple glow, is where the laser light is actually generated. It consists of a gas-filled glass tube along which a high voltage is set up. The voltage excites the atoms of gas so that they subsequently return to their original states by emitting photons. When these photons collide with other atoms they too are induced to emit photons of the same frequency. Eventually an avalanche of photons, the laser light, is emitted at one end of the tube.