Figure 2.3 — Reflection and transmission at a planar interface Figure 2.3 — Reflection and transmission at a planar interface. As (a) an s-polarised or (b) a p-polarised ray ri impinges onto a planar interface between two media with refractive indices ni and nt, it splits into a reflected ray rr and a transmitted ray rt.The arrows perpendicular to the rays represent the directions of the electric fields and n is the line perpendicular to the interface at the incidence point.