We have performed the first detailed calculation of the index of refra
ction of a gas (argon) for an atomic wave (sodium). The index depends
rapidly on the wave vector k(r) describing the relative motion and exh
ibits two interesting effects: Breit-Wigner-type resonances at very lo
w k(r) values, and intense glory oscillations over a large range of k(
r) values. A recent experiment has measured this index of refraction,
and our calculation using a very accurate sodium-argon potential and t
aking into account the thermal motion of argon, agrees quite well with
the experimental result.