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This usually happens when we see a car speeding on a movie screen or television. The explanation is that the screen (both in movies and TV) “blinks”, that is, between one frame and another, it goes off for a microsecond. As we only see the image when an object is illuminated, it is as if it also blinks before our eyes. This is barely noticeable, but when the object in question is also rotating at a constant rate, what physicists call a strobe effect can occur.
What the hell is this? Calm down, let’s go. Imagine a fixed point on the wheel of a car. If the screen flashes a little faster than the wheel’s rotation frequency, we’ll see that point again before it completes a full turn. The sequence of these points, one always a little behind the other, gives the impression that the wheel is turning backwards. Did you understand? If the screen blinks slower than the tire turns, the impression is that the wheel rotates forward. If the two frequencies coincide, the dot will always be seen in the same place – there it seems that the tire is stationary. In Western movies where wagons appear, the stroboscopic effect is even more evident, as the rims of the old wooden wheels are well defined.
Off screen, this optical illusion can happen when objects are illuminated by fluorescent or gas discharge lamps, which also “blink”. So at night, in these lighting conditions, you can also see car wheels turning backwards. During the day, as sunlight is constant, the effect is much rarer.
Pure illusion
The strobe effect is everywhere
FAN TEST…
Who has never seen a fan spin slower, almost stopping? This experiment can be done in a room illuminated with cold light and it is very clear if one of the helices is marked with a pen.
…FROM THE TAP…
When the drippings from a faucet are illuminated by a strobe lamp (like those in nightclubs), we may have the sensation that the drops are returning to the faucet or standing still in the same place — it depends on the frequency of the light and the drops.
…AND THE FACTORY
Inside fluorescent-lit factories, machines and engines can appear to stand still or run at low rpm. To avoid accidents at work in these places, special lamps were developed to minimize this effect.