Only when lit by electrical lamps, table fans seem to rotate forwards and backwards. Wheels of cars running on the roads also give a similar illusion. These are a result of stroboscopic effect.
The illusion does not occur when the fan or wheel is lit by sunlight or candle light. This naturally leads us to a fundamental difference between these light sources.
Electrical lamps emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for 10 milliseconds, goes off for the next 10 milliseconds, and the process repeats 50 times a second). But we don’t see the on-off processes because of our eye’s persistence of vision. (The eye has the ability to retain the impression of an image for short time even after the image has disappeared). But sunlight and candle light are continuous without periodically going off and on.
Hence when the light goes off, the eye involuntarily retains the image of the fan blades’ or wheel spokes’ position.
Again when the light comes on, the fan’s blades would have moved to a new position and the eye records a new image. Depending on the speed the fan or wheel, the image retained by the eve gives us the illusion.
Supposing we make a fan rotate at a speed of 50 rpms and place it under an electrical lamp, the fan would appear to be stationary. If the speed is different from 50 rpm, the fan seems to rotate slowly forwards or backwards depending on the speed.
