where J → → is the probability current density. This gives the fraction of incident beam of particles that makes it through the potential. For three-dimensional problems, one would calculate the scattering cross-section σ σ , which, roughly speaking, is the total area of the incident beam which is scattered. Another quantity of relevance is the partial cross-section, σ σ l , which denotes the scattering cross section for a partial wave of a definite angular momentum eigenstate. These quantities naturally depend on k → → , the wave-vector of the incident wave, which is related to its energy by: