We investigate the plane-wave solution of Maxwell's equations for a stratified medium characterized by a slowly varying index n(z) . The study is carried out in the time domain. We show that the transmitted wave from a source generates reflected wavelets which are propagated in the opposite direction. The reflected wave may be interpreted as the propagation medium response to the source excitation. The medium is thus characterized by a transfer function and an impulse response exclusively dependent on the propagation velocity. Conversely, knowing the impulse response makes estimating the index n(z) possible.