We calculate the reflection probability for ultracold sodium atoms inc
ident on an evanescent-wave atomic mirror. For low enough energies the
reflection probability curves exhibit quantum effects that are sensit
ive to the long-range part of the potential and can therefore be used
to resolve Casimir retardation effects. We also explore the accuracy o
f model potentials approximating the exact atom-wall effective potenti
al. [S1050-2947(98)08811-8].