  We calculate the effect of the interaction between an optically active
material and a Bose-Einstein condensate on the collective oscillations of the
condensate. We provide explicit expressions for the frequency shift of the
center of mass oscillation in terms of the potential generated by the substrate
and of the density profile of the gas. The form of the potential is discussed
in details and various regimes (van der Waals-London, Casimir-Polder and
thermal regimes) are identified as a function of the distance of atoms from the
surface. Numerical results for the frequency shifts are given for the case of a
sapphire dielectric substrate interacting with a harmonically trapped
condensate of $^{87}$Rb atoms. We find that at distances of $4-8 \mu m$, where
thermal effects become visible, the relative frequency shifts produced by the
substrate are of the order $10^{-4}$ and hence accessible experimentally. The
effects of non linearities due to the finite amplitude of the oscillation are
explicitly discussed. Predictions are also given for the radial breathing mode.
