Why a wheel can appear to reverse
A wheel’s repeated spokes make its position ambiguous. If a flash arrives when one spoke has moved into nearly the place another occupied at the previous flash, the snapshots look almost identical. Your brain may interpret the small shift between them as motion in the opposite direction. This is a visual version of aliasing: when a changing pattern is sampled too slowly, its apparent movement can differ from its actual movement.
The flash timing sets the illusion
Imagine a wheel with evenly spaced spokes. If it turns by exactly one spoke-spacing between flashes, each glimpse shows the same arrangement, so the wheel seems still. If it turns slightly less than that, each glimpse shows the spokes just short of their previous positions. The wheel can then seem to creep backward. A slightly different timing can make it seem to move forward, or make its motion easier to notice.
The effect depends on the relationship between the wheel’s rotation, the spacing of its repeated features, and the interval between flashes—not simply on whether the flashes are fast or slow. Changing any one of those can change the apparent direction or speed.
What the eye is—and is not—seeing
The wheel does not actually stop or reverse. Each flash briefly makes its position visible; the visual system compares those glimpses and infers what happened between them. Because the flashes leave gaps, that inference can be wrong. With a steady, unbroken view, the intervening motion supplies clues that help reveal the wheel’s true direction. Under strobing light, the gaps become part of the illusion.
