Definition: Light Amplification by Stimulated Emission of Radiation: production of a highly monochromatic, coherent light. * Consists of a cavity, with mirrors at the ends facing each other. The cavity filled with lasable material i.e. one whose majority of atoms are capable of being excited to a semi-stable state by light or an electric discharge. The light emitted by atoms as they return to their ground energy state stimulates other excited atoms to radiate energy: this continually increases as light bounces backwards and forwards between the mirrors and power input to excite the material. * One mirror is slightly transparent: a beam of monochromatic, coherent emerges from this. * The beam can be a continuous wave or a brief pulse of energy. * If the mirrors are flat, the beam is collimated, if concave, the beam appears to emerge from a point source. * Lasable materials can be gas, liquid, semi-conductor crystal, etc.

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