ABSTRACT
When none of the rigorous approaches, analytic or numerical, lends itself to an exact solution, one is left with no other choice but to take resort to approximation methods. Sometimes however, the approximation methods are a preferred alternative. This is because (i) they offer simple analytic expressions which are easy to use. Thus, whenever they provide sufficient accuracy, it is desirable to employ them. (ii) The closed form expressions that they provide result in the development of better understanding of the processes involved, and they provide a deeper physical insight into the problem. (iii) Computationally demanding implementations sometimes become constrained by roundoff errors; particularly at large x or where large number of particles of various sizes and shapes are involved. (iv) Modelling a scatterer by a shape itself introduces an approximation. A true description of a shape in terms of a mathematical equation can only be a rare occurrence.
