According to the description of an atom interacting with the electromagnetic field, the absorption of light by the atom depends on the frequency of the incident photons. More precisely, the absorption is characterized by a Lorentzian of width &Gamma;/2 (for reference,  for common rubidium D-line transitions). If we have a cell of atomic vapour at room temperature, then the distribution of velocity will follow a Maxwell–Boltzmann distribution
where  is the number of atoms,  is the Boltzmann constant, and  is the mass of the atom. According to the Doppler effect formula in the case of non-relativistic speeds,
where  is the frequency of the atomic transition when the atom is at rest (the one which is being probed). The value of  as a function of  and  can be inserted in the distribution of velocities. The distribution of absorption as a function of the pulsation will therefore be proportional to a Gaussian with full width at half maximum
For a rubidium atom at room temperature,
Therefore, without any special trick in the experimental setup probing the maximum of absorption of an atomic vapour, the uncertainty of the measurement will be limited by the Doppler broadening and not by the fundamental width of the resonance.