A process by which a particle in the @G02704@ or an @E02257@ may undergo transition to a higher energy level by @A00036@ of a photon. For a given particle in the lower state of @P04855@ per second of such a transition in a field with a continuous spectrum is proportional to the @ST07480@ density of the @A00041@. The proportionality constant is termed the transition @P04855@ for absorption. The transition @P04855@ for @S06015@ is defined in a similar way for the reverse @R05062@ process that is induced by the same radiation field.