A cross section is therefore a measure of the effective surface area seen by the impinging particles, and as such is expressed in units of area. The cross section of two particles (i.e. observed when the two particles are colliding with each other) is a measure of the interaction event between the two particles. The cross section is proportional to the probability that an interaction will occur; for example in a simple scattering experiment the number of particles scattered per unit of time (current of scattered particles I r) depends only on the number of incident particles per unit of time (current of incident particles I i), the characteristics of target (for example the number of particles per unit of surface N), and the type of interaction. For Nσ ≪ 1 we have