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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