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Reverse rotation effect

Reverse rotation effect is an optical illusion caused by car wheels which appear to rotate backwards when reaching a certain velocity.

Reverse rotation of wheels usually only occurs in film (television or movies). It is caused by a strobe effect. A movie camera operates at 24 frames per second. If a wheel is turning at some multiple of 24 revolutions per second, the spokes will be in the exact same position every time the shutter opens. The wheel will appear to be motionless (though possibly blurred) upon viewing the film. If the wheel begins to slow down slightly, it does not have time to rotate to its original position before the shutter opens again. It instead appears to be rotating backwards.

Reverse rotation effect is also visible on car wheels illuminated by artificial lighting rather than sunlight. When powered by alternating current, gas discharge lamps (such as neon, mercury vapor, sodium vapor and fluorescent tubes) flicker at twice the frequency of the power line (that is, 120 times per second on a standard 60 cycle line). In each cycle of current the power peaks twice (once with positive voltage and once with negative voltage) and twice goes to zero, and the light output varies accordingly. Although 120 flickers per second is too fast for humans to perceive directly, such lamps can produce stroboscopic effects.

The phenomenon is a prime example of aliasing, where a periodic event is sampled at a frequency below the Nyquist frequency.