Why Do Wheels Sometimes Appear To Be Reversing?

This visual phenomenon, which we encounter many times, is mostly seen in high-class vehicles after a fast take-off. When the car takes off, everything seems normal, but as the speed increases, the abnormality begins.

When it comes to a certain point, the wheels seem to turn more slowly at first, then appear to stop for a short time, and then seem to turn back. This situation is called the “wagon wheel effect” or the “stroboscopic effect”.

Why do we see it most often in cinemas?

Cinemas consist of a large number of photographs shown in succession. That’s why cinemas are also called “motion pictures” in English. The image flows in the middle of 25-60 seconds per second on the screen, these frame numbers may vary according to the technique. Since our brain fills the spaces in the middle, we perceive the landscape uninterruptedly.

The first motion picture

If the image we watch is 30 fps, that is, if we see 30 frames in 60 seconds, it means there are 2 split seconds in the middle of two photos. If the wheel of the car is faster than the frame speed in this time period, that is, if the type around itself is in the middle of 1-2 splits, the photos appearing in the frames in the cinema 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 automobile wheels, it can actually be seen in all fast rotating propellers.

The reason why the rotor of the helicopter in this image appears to be standing still is because the frame rate and the rotation speed of the propeller are equalized. So one full variant of the propeller is equal to the succession speed of the two frames in the image.

In this drawing, the event is given clearly:

This visual illusion, which we often see in images, can sometimes be witnessed in real life.

The reason why the fan in the image seems to stop even though it is spinning is not related to the image, but to the frequency of the light that illuminates the environment. The light is actually not always, it flashes very quickly in intermittent flashes. We do not see the moments when the light is cut off, but our eyes always perceive it. This blink rate changes according to the frequency of the light.

Here, the time difference in the middle of the light flashes that occur at certain intervals may cause the movement of the propeller to be perceived differently. A fluorescent bulb operating with 50 hertz alternating current blinks 50 times per second. If the fan beats at full speed in each flash, the fan will be detected as stopped. It is a precedent logic with the speed of recording images in cameras.

The fact that this situation, which we call the “stroboscopic effect”, is experienced not only in cinemas but also in reality can lead to some deadly dangers.

Since rotating objects will be perceived as not rotating, someone may reach into 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