Retrograde Motion Illusion for an Electric Fan

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This Demonstration shows the rotation of an electric fan in slow motion—by about a factor of 10—for computer display compatibility. The fan rotates clockwise. A conventional incandescent light source is somewhat stroboscopic, due to the 60-cycle alternating current powering it.


Certain combinations of rotation rate and strobe frequency produce the illusion of retrograde motion, with the fan apparently turning counterclockwise. If the two frequencies are appropriately in synch, the fan can even appear to be motionless.


Contributed by: S. M. Blinder (March 2011)
Open content licensed under CC BY-NC-SA



Snapshot 1: At relatively low RPM, see clockwise rotation.

Snapshot 2: Speeding up rotation, get retrograde illusion.

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