  The onset of retardation effects in atom-wall interactions is studied. It is
shown that the transition range from the 1/z^3 short-range (van der Waals)
interaction to the 1/z^4 long-range (Casimir) retarded interaction critically
depends on the atomic properties and on the dielectric function of the
material. For simple non-alkali atoms (e.g., ground-state hydrogen and
ground-state helium) interacting with typical dielectric materials such as
intrinsic silicon, the transition to the retarded regime is shown to proceed at
a distance of about 10 nm (200 Bohr radii). This is much shorter than typical
characteristic absorption wavelengths of solids. Larger transition regimes are
obtained for atoms with a large static polarizability such as metastable
helium. We present a simple estimate for the critical distance,
z_cr=137*(\alpha(0)/Z)^(1/2) atomic units, where alpha(0) is the static
polarizability (expressed in atomic units) and Z is the number of electrons of
the atom.
