In regions where a step potential or potential barrier occurs, the probability current is related to the transmission and reflection coefficients, respectively T and R ; they measure the extent the particles reflect from the potential barrier or are transmitted through it. Both satisfy: T + R = 1 , where T and R can be defined by: T = | j t r a n s | | j i n c | , R = | j r e f | | j i n c | , where j inc, j ref, j trans are the incident, reflected and transmitted probability currents respectively, and the vertical bars indicate the magnitudes of the current vectors. The relation between T and R can be obtained from probability conservation: j t r a n s + j r e f = j i n c .