We study the dynamics of ultracold atoms in a periodic optical potenti
al submitted to a constant external force. Bloch oscillations in the f
undamental and first excited bands of the potential are observed. In a
ddition to a solid-state analysis we give a quantum-optics interpretat
ion of this effect in terms of photon exchanges between the atoms and
the laser waves. The dynamics of Bloch oscillations are equivalent to
a sequence of adiabatic rapid passages between momentum states and can
be described using the dressed-atom approach. We demonstrate efficien
t and dissipation-free acceleration of atoms by coherent transfer of a
large number of photon momenta (approximate to 100). This technique p
roduces atomic beams with a very small longitudinal velocity spread th
at may find applications in atom optics and precision measurements.