Why do wheels appear to spin backwards at high speeds?

While standing on the footpath or while watching movies,
Have you ever wondered?, Why the wheels of the cars appear to spin backward when they are at high speeds?

Wheels

Let me explain why this happens with a short example. Suppose a car is moving forward and with time it is gaining momentum and speed. The wheels of the car moving forward, at first will appear to spin in one direction and as the speed of the car increases, Its wheels will also rotate faster. But then, something weird happens.

At a certain point, the spin of the wheels appears to get slower and at some point, the rotation stops. But when it resumes, we see that the spin is in opposite direction. Due to this type of rotation, the car should be moving backward, isn’t it?  But the car is moving forward. This phenomenon is known as The Wagon Wheel Effect. Most of the people, including you, are likely to see wagon wheel effect in movies or televisions.  Let us see why the effect appears to our eyes like this?

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In movies or TV’s the cameras record footage by capturing a series of images in a quick session and not recording it continuously. The cameras capture the images at a specified rate called “frame rate”. Many movie cameras have a frame rate of 24 frames per second,  and when the frame rate of this camera matches with the frequency of a wheel’s spin(i.e. 24 revolutions per second), each of the wheel’s spoke completes a full revolution every 1/24 seconds, and due to this it ends up in the same position every time a frame is captured by the camera.

So, we can say that when a wheel seems to spin in the direction opposite to that of its actual rotation is because each spoke has come up a few degrees shy of the position it occupied when it was last imaged by the camera. It is sometimes referred to as a reverse-rotation effect. But if the spoke somehow over-shoots, the wheel will appear to rotate in the right direction, but very, very slowly. The appearance and effect of the effect also depend upon the exposure time of the camera and also the design of the wheel.

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