Surface Plasmon Resonance

Hi, this is particularly for optics folks. I am trying to understand SPR - surface plasmon resonance and I prefer to use mathcad to see exactly what is going on. As can be seen in the attached, the reflectivity function (neglecting phase) has an infinite value when the two angles subtracted = 90 degrees. Is there anyway to view this on a surface plot by capturing this and just put a large number, or removing this.

Thanks
Jason