  The interaction between a polarizable particle and a reflecting wall is
examined. A macroscopic approach is adopted in which the averaged force is
computed from the Maxwell stress tensor. The particular case of a perfectly
reflecting wall and a sphere with a dielectric function given by the Drude
model is examined in detail. It is found that the force can be expressed as the
sum of a monotonically decaying function of position and of an oscillatory
piece. At large separations, the oscillatory piece is the dominant
contribution, and is much larger than the Casimir-Polder interaction that
arises in the limit that the sphere is a perfect conductor. It is argued that
this enhancement of the force can be interpreted in terms of the frequency
spectrum of vacuum fluctuations. In the limit of a perfectly conducting sphere,
there are cancellations between different parts of the spectrum which no longer
occur as completely in the case of a sphere with frequency dependent
polarizability. Estimates of the magnitude of the oscillatory component of the
force suggest that it may be large enough to be observable.
