The quantum mechanical treatment of atom describes a single atom. Hence, the atom field interaction discussed in the last chapter describes the interaction of a single atom with a classical electromagnetic field. In any experiment, under usual circumstances, we have a large number of atoms interacting with the field. The observed result is an average over the effect of interaction of the electromagnetic field with all these atoms. The atoms may not all be in the same quantum mechanical state. For the atom in one quantum mechanical state, the observed value is an expectation value which is an average over a large number of measurements of the same physical variable in the same state. The atom is described by a set of eigenstates φn . At any instant the general wave function of an atom is () | ψ ( t ) 〉 = ∑ n c n ( t ) | ϕ n 〉 . https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492396/2543d099-b354-4900-b875-fd425f903a05/content/eq281.tif"/>