File:StraightLineReinforcement.PNG

English: This "explore all paths" straight line^[1] in the "single wave-packet picture" uses the color (hue) of each pixel to represent the deBroglie phase-angle, and yields clues to both refraction, diffraction, and wavefunction collapse.

In particular here light willing to explore all paths is seen to arrive mainly via a (dashed brown) straight line because it's from that direction alone that the deBroglie phases "pile up" so as to reinforce rather than cancel out. The figure also provides insight into how photons automatically choose the path of least time, and why trajectory modifications are most significant if they attempt to restrict the trajectory to a width of less than one wavelength (dotted line envelope).

The vertical series of dots at {2,y} shows the phase contribution at {2,0} from trajectories passing through the bend-line at {1,y}. Outside of the wavelength-width band centered around the straightline at y=0, the phases turn a bit orange (contributing the 45 degree added-length phase-shift associated with the Cornu spiral) and then become random.

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 which are straight, except for a bend at the half-way point between an emission point at left (x=0) and a detection point at right (x=2).