In the St. Catharines Standard “Search Engine” column on May 8, Physics chair Edward Sternin explained why wheels on fast-moving vehicles seem to be going backward. The oddity is called the stroboscopic effect, he said.
The rest of his response:
Sternin explained in an email that light receptors in our retinas need about 30 milliseconds to detect light that comes into the eye and send the image to the brain.
If the image changes faster — more than 30 to 40 times per second — we don’t notice it.
Movies and TV programs rely on that inability to create an illusion of smooth motion from a sequence of snapshots.
Sternin said if you were to slow down a movie and look at it frame-by-frame you would see the little jumps a moving object performs from one frame to the next.
But when the same sequence passes in front of your eyes at the normal rate of 24-to-30 frames per second, your eyes and your brain perceive the sequence of flashes as one continuous motion.
A rotating wheel is a special case.
That’s because the motion is periodic, with the small markings on the side of the wheel repeating their position with every full turn.
Sternin said if a wheel rotates exactly one full turn in the time it takes for the next frame to be captured, every shot in a sequence would look identical to the previous one — the wheel would look like it wasn’t moving at all.
If the wheel rotates almost a full turn, such as from 12 o’clock to 9 o’clock, our brains don’t recognize it as a 3/4 rotation forward but as a 1/4 rotation backwards.
This happens only at very high speeds, about 200 kilometres an hour, when wheels turn at the same rate as the frames of the TV, about 30 times per second.
Sternin said the effect is easier to achieve in wheels with a repeat pattern, like four or five mounting bolts or six to eight spokes of the wheel in a wagon.