Interaction with Atoms

Interaction of an atom with radiation can be analyzed in three ways. They are 1) An atom makes a spontaneous transition from an excited state to a state of lower energy by emitting a photon which is the quantum of the electromagnetic field. 2) An atom absorbs the photon from a beam of radiation which makes a transition from a state of lower to a state of higher energy. 3) Atoms can also emit photons through the influence of an applied radiation field.