Due to time-of-flight effects the line-shapes of dark/bright absorptio
n resonances and of the magneto-rotation spectrum (non-linear Faraday
effect) observed in the forward scattering of resonance radiation in a
lkali vapors differ from absorptive and dispersive lorentzians. We pre
sent a simple model which takes the finite interaction time of vapor a
toms with a gaussian laser beam into account and compare the theoretic
al results with experimental data obtained on the Cs D2 line.