  We consider the forces exerted by a pulse of plane-wave light on a single
atom. The leading edge of the pulse exerts a dispersive force on the atom, and
this modifies the atomic momentum while the atom is enveloped in the light. The
standard view of the optical dipole force indicates that red-detuned light
should attract the atom towards high intensity. This should increase the
average momentum per photon to $\textbf{p}_{0} n$, where $\textbf{p}_{0}$ is
the photon momentum in free space and $n$ is the average refractive index due
to the presence of the atom in the light. We show, however, that this is the
wrong conclusion and that the atom is in fact repelled from the light by the
dispersive forces, giving the photons a momentum $\textbf{p}_{0} /n$. This
leads us to identify Abraham's optical momentum with the kinetic momentum
transfer. The form due to Minkowski is similarly associated with the canonical
momentum. We consider the possibility of demonstrating this in the laboratory,
and we note an unexpected connection with the Aharonov-Casher effect.
