optical pumping, in physics, the use of light energy to raise the atoms of a system from one energy level to another. A system may consist of atoms having a random orientation of their individual magnetic fields. When optically pumped, the atoms will undergo a realignment of individual magnetic fields with respect to the direction of the light beam; that is, there will be a rearrangement of magnetic energy levels; and, in some cases, the bulk of the atoms will be oriented to generate a magnetic field throughout the system.

More familiarly, optical pumping takes place in laser action, in which the energy of electrons is raised by their absorption of photons of light. The electrons remain at the higher energy level until they are triggered to release their stored energy in the form of a laser beam, in which the light waves all have the same wavelength and vibrate in unison.