Explore all pathsEdwin F. Taylor, Stamatis Vokos, John M. O'Meara, and Nora S. Thornber (Mar/Apr 1998) "Teaching Feynman's Sum Over Paths Quantum Theory", "Computers in Physics" "12", 190-199, link. refraction yields clues to the angles in Snell's law in going from a faster longer-wavelength medium (left) to a slower shorter-wavelength medium (right), as well as the photon's ability to automatically choose the path of least time. Although this might at the left remind you of a Huygen's wavefront, it is less that than an attempt to illustrate the Feynman propagator's "sum over paths" for a single-photonRichard P. Feynman (1985,2006) "QED: the strange theory of light and matter" (Princeton University Press, Princeton NJ).Michael E. Peskin and Daniel V. Schroeder (1995) "An introduction to Quantum Field Theory" (Perseus Books, Reading MA).. It considers only that subset of paths from emission point at left (x=0) to a detection point at right (x=2) which are straight except for a bend at the tilted interface between dissimilar media.