The classical quantity | α | appearing in the classical expression for the energy of a field mode is replaced in quantum theory by the photon number operator a a. The fact that: [ a , a † † a ] ≠ ≠ 1 implies that quantum theory does not allow states of the radiation field for which the photon number and a field amplitude can be precisely defined, i.e., we cannot have simultaneous eigenstates for a a and a. The reconciliation of wave and particle attributes of the field is accomplished via the association of a probability amplitude with a classical mode pattern. The calculation of field modes is entirely classical problem, while the quantum properties of the field are carried by the mode "amplitudes" a and a associated with these classical modes.