  We present an elementary discussion of the momentum transferred by an
electromagnetic wave propagating in a dispersive medium. Our analysis is based
on Minkowski's electromagnetic momentum density which have been recently seen
to be consistent with a fully covariant expression of the energy-momentum
tensor of the electromagnetic field in a dispersive medium and with all the
experimental evidence. We show that the medium may be either pulled or pushed
as the electromagnetic pulse enters in it depending on the value of the
frequency.
