Spinning wheels on a car in a commercial or movie may appear to be spinning backwards due to optical illusions, including animation principles and the strobe effect. The brain can be fooled by limited information and assumptions based on what it knows.
Many people notice that the spinning wheels on a car shown in a commercial or movie appear to be spinning backwards even though the car is clearly moving forward. It is especially noticeable when the car is just beginning to accelerate or slow down. During a long shot of the car driving down an open road, its spinning wheels may even appear to stop and then spin backwards. There are actually several different optical illusions at work that make spinning tires appear to be spinning in reverse.
There is a fundamental difference between actual motion and the type of animation used to create the illusion of motion on film or video. When the wheels of a real car begin to move forward, our eyes can usually follow the movement of the individual spokes until the car reaches a certain speed and they become blurry. After that, our eyes can only glimpse certain spokes if they are illuminated by an outside light source. This provides our brain with enough information to recognize the forward motion of actual spinning tires.
Animation, however, works on a different principle. The actual movement of the car is only represented by a series of still images that move at 24 or possibly 30 frames per second. Our eyes see the same spinning edges, but now the information enters the brain at a different rate. If an individual spoke in a series of animated images appears in a different position, the brain interprets this as moving in a certain direction. If a radius appears lower in subsequent frames, it will appear to recede in our mind. Even if the actual wheel is clearly moving forward, the individual spokes of the spinning rim may appear to be moving in reverse.
In addition to the brain’s interpretation of moving spokes, our eyes can be fooled by an optical illusion called the strobe effect. A strobe is a device that can be calibrated to send pulses of light at a designated rate per second. If the light from a strobe is focused on a fast-moving object, such as the rotating rim of a car, individual elements of that object may appear to be stationary, moving forward or backward. In a car commercial, an outside light source like the sun or movie lamps could create a strobe effect as individual rays are illuminated. Even though the actual tires are clearly moving forward, the strobing effect can create an illusion of recoil or even a complete stop, just as dancers can appear motionless when illuminated by a strobe light.
When information is limited, the brain often makes assumptions based on what it knows. In the case of spinning wheels, the relative position of the individual spokes and the light source that makes them visible can combine to create an optical illusion of backward movement.
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