  Using the general expressions for level shifts obtained from the master
equation for a small system interacting with a large one considered as a
reservoir, we calculate the dispersive potentials between an atom and a wall in
the dipole approximation. We analyze in detail the particular case of a
two-level atom in the presence of a perfectly conducting wall. We study the van
der Waals as well as the resonant interactions. All distance regimes as well as
the high and low temperature regimes are considered. We show that the
Casimir-Polder interaction can not be considered as a direct result of the
vacuum fluctuations only. Concerning the interaction between the atom and the
wall at high temperature, which show that a saturation of the potential for all
distances occurs. This saturated potential coincides exactly with that obtained
in the London-van der Waals limit.
