Why does the helicopter propeller seem to rotate in the opposite direction to the actual direction?

The reason why the helicopter rotor looks like it is reversing is not caused by visual persistence.

In fact, if you look at a high-speed rotating object, you can't see the rotating object, only a circular fuzzy shadow.

The reason why it looks like reversing is mainly because of the camera's frame rate.

Whether it is a film camera or a digital camera, the picture is not absolutely coherent, but is played frame by frame.

There is a certain frame rate, that is, the number of pictures displayed per second or the number of pictures taken per second.

If the rotation period of the rotor is consistent with the frame rate of the camera, during the time between instantaneous pictures taken each time,

The rotor has just made one revolution, and it has returned to the position of the previous frame, so the position of the rotor has not changed at every moment, so it seems that the rotor is static and does not rotate on the screen.

If the rotation period of the rotor is close to the frame rate of the camera, but slightly slower than the frame rate of the camera, it will lead to the following situations:

After the first frame is taken, a certain position of the rotor is fixed on the screen, and then the rotor continues to rotate. In the next frame, the rotor rotates less than one revolution, and the position is slightly behind the previous frame. Then, the rotor position of each frame is slightly behind the previous frame, so it seems that the rotor is continuously rotating backwards.

Similarly, if the rotation period of the rotor is close to the frame rate and slightly faster than the frame rate, it seems that the rotor is slowly rotating forward on the screen.