English: Explore all paths^[1] 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-photon^[2][3]. 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.