This is due to an effect known as ‘aliasing’ and is most likely to be observed on TV due to the frame rate of the camera filming it. Video cameras work by capturing lots of still images in a very short space of time. For example, television 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 with four spokes at right angles to each other, 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 then your brain will interpret the spoke as having moved anti-clockwise from the 12 o’clock position to the 11 o’clock position, making the wheel appear like it is rotating backwards.
This effect can also be seen quite well under a fast-moving strobe light, as essentially the strobe is doing the same thing as the camera and giving you lots of snapshots of an image. Under certain conditions, street lights can highlight this effect as they are constantly flickering on and off about 50 times a second due to the alternating current.
Author: Rik Sargent, Science Museum