I'm having an issue in comprehending the minimum deviation offered by a prism. The fact that we could use the symmetry argument about the angle of incidence and angle of emergence being equal for minimum deviation make sense to me but I couldn't understand why we can be so sure of exactly 1 such point. I obtained an explicit function of the deviation(any) in terms of the angle of incidence alone but I can't analyse it with my current knowledge of inverse functions. I'm in class 12. Thank You.