We discussed the propagation of plane waves in unbounded space in Chap.  7 . In this chapter, we consider the situation where a plane wave encounters an object in the shape of a flat infinite-extent surface (as shown in Fig. 8.1). The entire space is then divided into two regions; a region with medium 1, and a region with medium 2. The surface separating the two regions is called an interface. In this case, part of the incident wave gets reflected by the object, and part of it gets transmitted.