The distance,  at which  can be considered as the distance at which the atom will come across a troublesome discontinuity in the potential. This is the point at which the WKB method truly becomes nonsense. The condition for efficient quantum reflection can be written as .
This refers to the point where the Wentzel-Kramers-Brillouin (WKB) approximation fails to accurately describe the behavior of an atom encountering a potential barrier. The condition for efficient quantum reflection is given by .