Fans of cars and films dedicated to the racing genre have probably noticed that when the transport rushes forward at high speed on the screen, its wheels can rotate in the opposite direction. It is easy to guess that this is a visual deception, and in fact the wheels spin in the same direction as the car goes. But what causes this effect?

The phenomenon in the cinema

The effect of reverse rotation of the discs often appears in the cinema. Its appearance depends on the frame rate of the camcorder and the speed of the vehicle. Suppose a camera writes at a standard frequency of 24 frames per second. It turns out that it fixes the position of the object every 41.6 milliseconds. Now imagine that at this moment a moving car is being filmed.

If the frequency of rotation of the wheels coincides with the interval between frames and is equal to 41.6 mls, then the video discs will be motionless, because each frame will fix them in the same position. When the car’s speed is faster, the wheels on the roller will rotate in the same direction as the car is moving, but do not do it as fast as he does. And if the discs make a full revolution slower than the pause between frames, then a deceptive effect of their movement in the opposite direction will be created. Just a movie camera with each frame will fix the position of the disk, which did not have time to complete a complete revolution.

Even during the filming of large-scale chases, drivers rarely develop a high speed for safety reasons, so this effect often appears on the screen.

Interesting fact: a similar anomaly can be seen in the cinema not only on the wheels of a car, but also on other rotating objects. It is called the strobe effect.

Life phenomenon

A similar effect can also be observed at night, when the car passes under the electric lights that illuminate the road. Their lamps operate using the principle of low-frequency pulsation, emitting light with constant pulses. If you carefully look at the included night light, you will notice that its lamp pulsates quickly, constantly becoming brighter or dimmer.

And when a person looks at objects illuminated by their light, his eye captures the picture only at the moment of the next impulse, when a beam of light diverges in space. Accordingly, the same principle works here as in the cinema, only the frame rate of the movie camera is replaced by the light pulsations of the lamppost.