This visual phenomenon, which we encounter many times, is usually seen in high-class vehicles after a quick take-off. When the car takes off, everything seems normal, but as the speed increases, the abnormality begins.
At a certain point, the wheels seem to spin more slowly, then seem to stop for a short while, and then seem to turn back. This is what is called the “wagon wheel effect” or the “stroboscopic effect”.
Why do we see it most often in movies?
Movies consist of multiple pictures that are displayed in succession. That’s why movies are also called “motion pictures” in English. Between 25-60 images are flowing on the screen per second, these frame numbers may vary depending on the technique. We perceive the image uninterruptedly because our brain fills in the spaces in between.
If the video we watch is 30 fps, that is, if we see 30 frames in 60 seconds, it means there are 2 splits between two images. If the car wheel is faster than the frame rate in this time period, that is, if its rotation is between 1-2 seconds, the pictures appearing in the frames in the movie will be different from the real ones. You will understand the event more clearly in the following visual explanation.
Although this situation is mostly seen in car wheels, it can actually be seen in all fast rotating propellers.
The reason why the propeller of the helicopter in this image seems to stop is because the frame rate and the rotation speed of the propeller are equalized. So one full rotation of the propeller is equal to the succession rate of two frames in the video.
The event is given clearly in this drawing:
This visual illusion, which we usually see in videos, can sometimes be witnessed in real life.
The reason why the fan in the video seems to stop even though it is spinning is not related to the video, but to the frequency of the light that illuminates the environment. The light is actually not continuous, it flashes very quickly in intermittent flashes. We do not see the moments when the light is cut off, but our eyes perceive it constantly. This blink rate changes according to the frequency of the light.
Here, the time difference between the light flashes that occur at certain intervals can cause the movement of the propeller to be perceived differently. A fluorescent bulb powered by 50 hertz alternating current blinks 50 times per second. If the propeller rotates fully in each flash, the fan will be perceived as stopping. It’s a similar logic to the image recording speed in cameras.
The fact that this situation, which we call the “stroboscopic effect”, is experienced not only in movies but also in reality can lead to some deadly dangers.
Since rotating objects will be perceived as not spinning, someone may poke their hand at work tools with dangerous propellers. For this reason, the rules in the internal regulation of electrical installations are arranged accordingly. Lighting elements in workplaces are connected to different phases. In this way, overlapping of frequencies is prevented and illusions are not experienced.
It is difficult to see the “stroboscopic effect” in daylight because there is more than one frequency of light in this light.
Otherwise there would have been a lot of trouble.
Sources: TUBITAK, Live Science, Erciyes University, Yıldız Technical University
