Light Amplification by Stimulated emission of Radiation. A laser is a cavity, with plane or spherical mirrors AT the ends, that is filled with laser able material. This is any material - crystal, glass, liquid, dye or gas - the atoms of which are capable of being excited to a semistable state by Light or an electric discharge. The light emitted by an atom as it drops back to the Ground state releases other nearby, excited atoms, the light being thus continually increased in Intensity as it oscillates between the mirrors. IF one mirror is made to transmit 1 or 2 percent of the light, a brilliant beam of highly monochromatic, Coherent Radiation is emitted through the mirror. If plane mirrors are used, the beam is highly collimated. With concave mirrors, the beam appears to emerge from a point Source near one end of the cavity.