sentences:  Thus it is clear that when an electric field is applied to the atom, the dipole of the atom oscillates according to driving frequency and not natural frequency of the atom.If we also look at the positive frequency component of the electric field, 
  formula_30
  we can see that the emitted field is the same as the absorbed field other than the difference in direction, resulting in the spectrum of the emitted field being the same as that of the absorbed field.The result is that the two-level atom behaves exactly as a driven oscillator and continues scattering photons so long as the driving field remains coupled to the atom.