  The interaction between a focused beam and a single classical oscillating
dipole or a two-level system located at the focal spot is investigated. In
particular, the ratio of the scattered to incident power is studied in terms of
the oscillator's scattering cross section and the effective focal area. Debye
diffraction integrals are applied to calculate it and results are reported for
a directional dipolar wave. Multipole expansion of the incident beam is then
considered and the equivalence between this and the Debye diffraction approach
is discussed. Finally, the phase change of the electric field upon the
interaction with a single oscillator is studied.
