That bright shaft is a trick of traffic, not a blade clearing a path through vapor. Fog does not yield. Solar photons strike suspended droplets and are redirected toward the observer's eyes, which makes their route visible without moving the water aside. The air looks occupied. What seems a solid column is Mie scattering, exposed only where viewing geometry favors sight.
Fog is less a curtain than a crowded exchange, because every droplet redirects light and removes it from its original route. No tunnel exists. For droplets roughly comparable to visible wavelengths, Mie scattering sends much of the light forward, yet it also throws photons sideways and backward, while repeated encounters produce extinction through scattering and absorption. Edges deceive the eye. Against a dark background, contrast makes an untidy statistical field appear sharply bounded, and the eye reads its brighter region as a beam with a beginning and an end.
The image remains fragile: move your head, change the Sun's angle, or thin the fog, and the shaft can weaken, broaden, or vanish. Nothing gets swept aside. A sunbeam is selective photon delivery, governed by droplet concentration, path length, and contrast rather than by a cleared passage. Still, it feels solid. Between light, matter, and sight, the fog offers a column that exists only at the instant it is seen.
