We will perform the calculations in three conceptually differen ways: (i) by the usual way of matching the tangential components of the fields inside and outside the slab, (ii) by using the fact that we have one upgoing and one downgoing wave in the medium, and (iii) by the multiple scattering method. The three methods obviously lead to the same final result which is a matter of consistency. But each of these methods can give rise to better understanding of the concepts involved and the resulting phenomena.