  We analyze the propagation of an incident electromagnetic wave in a
purely-time modulated medium. Precisely, we assume that the permeability is
unchanged while the permittivity has a multiple-step profile in time and
uniformly constant in space. For this, we use time-dispersive Lorenz's model
with time-dependent plasma frequency with highly concentrated values near the
centers of the steps' intervals. Under certain regimes linking the number of
steps to the contrasts of the permittivity, we can generate effective
permittivity having positive and high values on a finite interval of time which
behaves as a 'wall' that kills the forward waves and keeps only the backward
waves (i.e. enabling full reflection). But this happens if these high values
are away from a discrete set. If these high values are close to the elements of
this discrete set, then the effective medium behaves as a 'well' that absorbs
all the forward waves and hence there is no backward waves (i.e. enabling full
transmission). Such results are reminiscent to the 'wall' and 'well' effects
known in the quantum mechanics theory.
