Although the principles of quantum mechanics apply to any particles, usually the term "quantum reflection" means reflection of atoms from a surface of condensed matter (liquid or solid). The full potential experienced by the incident atom "does" become repulsive at a very small distance from the surface (of order of size of atoms). This is when the atom becomes aware of the discrete character of material. This repulsion is responsible for the classical scattering one would expect for particles incident on a surface. Such scattering is diffuse rather than specular, and so this component of the reflection is easy to distinguish. Indeed, to reduce this part of the physical process, a grazing angle of incidence is used; this enhances the quantum reflection. This requirement of small incident velocities for the particles means that the non-relativistic approximation to quantum mechanics is all that is required. So far, one usually considers the single-dimensional case of this phenomenon, that is when the potential has translational symmetry in two directions (say formula_1 and formula_2), such that only a single coordinate (say formula_3) is important. In this case one can examine the specular reflection of a slow neutral atom from a solid state surface . Where one has an atom in a region of free space close to a material capable of being polarized, a combination of the pure van der Waals interaction, and the related Casimir-Polder interaction attracts the atom to the surface of the material. The latter force dominates when the atom is comparatively far from the surface, and the former when the atom comes closer to the surface. The intermediate region is controversial as it is dependent upon the specific nature and quantum state of the incident atom. The condition for a reflection to occur as the atom experiences the attractive potential can be given by the presence of regions of space where the WKB approximation to the atomic wave-function breaks down. If, in accordance with this approximation we write the wavelength of the gross motion of the atom system toward the surface as a quantity local to every region along the formula_3 axis,  where formula_6 is the atomic mass, formula_7 is its energy, and formula_8 is the potential it experiences, then it is clear that we cannot give meaning to this quantity where,  That is, in regions of space where the variation of the atomic wavelength is significant over its own length (i.e. the gradient of formula_10 is steep), there is no meaning in the approximation of a local wavelength. This breakdown occurs "irrespective of the sign of the potential", formula_8.