A laser produces a beam of monochromatic light in which all the waves are in phase or are coherent.
Lasers contain four primary components regardless of style, size or application.
Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. Lasing Action Light is produced when energy is applied to an atom raising an electron to a higher unstable energy level. This electron will then randomly return to its stable energy level by releasing a photon of light. Light produced in this manner is called incoherent light and has many different wavelengths, directions and phases, depending on a number of factors. When energy is applied to a laser active medium electrons are raised to an unstable energy level then spontaneously decay to a lower relatively long-lived metastable state.