WHY DO CAR WHEELS LOOK LIKE THEY’RE SPINNING BACKWARDS AT HIGH SPEEDS?

Sam Peters

This is due to an effect known as ‘aliasing’, often seen on TV.

Video cameras work by capturing lots of still images in a very short space of time. For example, TV cameras capture roughly 50 frames a second. This is quite sufficient to fool our eyes and brain into thinking we are seeing a continuous moving image. Now imagine a wheel and its spokes, and focus on the spoke in the 12 o’clock position. If, by the time the next frame captures an image, that spoke has moved clockwise almost one whole revolution to 11 o’clock, your brain will interpret the spoke as having moved anticlockwise, making the wheel appear as if it is rotating backwards. This effect can also be seen under a strobe light, as the strobe is doing the same thing as the camera, giving you lots of snapshots of an image. Under certain conditions, streetlights can highlight this effect as they are constantly flickering on and off about 50 times a second due to the alternating current.