A wheel that we know is spinning forward rapidly can sometimes seem like it is spinning backwards. The reason for this interesting situation, which is generally seen in movies, is related to the number of frames per second (fps). Let’s go into detail:
This is what we come across so many times. visual phenomenon it usually appears on high-end 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 turn more slowly, then stop for a short while, and then seem to turn back. Here’s to this situation “wagon wheel effect” or “stroboscopic effect” is called.
Why do we see it most often in movies?
Movies are made up of a large number of pictures that are shown 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. Because our brain fills in the gaps. we perceive the image without interruption.
first motion picture
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 pictures. 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 often seen in car wheels, it can also be seen in all fast rotating propellers.
The fact that the propeller of the helicopter in this image appears to be standing still The reason is that the frame rate and the rotational 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.
In this drawing, the event is given clearly:
It is possible to witness this visual illusion, which we usually see in videos, in real life as well.
The reason why the fan in the video seems to stop even though it is spinning is not related to the video, it is related to the frequency of the light that illuminates the environment. Light is not actually continuous, it moves very quickly. flashes with 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’s what pops up periodically time difference between flashes of light may 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.
rotating objects as it will be perceived asSomeone can get their hands on 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 be a lot of problems.
Sources: TUBITAK, Live Science, Erciyes University, Yıldız Technical University
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