I Didn’t Know That . . .

Q: Why do the wheels of some cars in television shows appear to be turning backward while the car is going forward?

A: This phenomenon, most commonly seen with stagecoaches in motion pictures, arises because television and movies are produced by a series of still photos that, observed in rapid succession, give the appearance of movement. If the TV is scanning photos at 50 frames per second and a wheel is rotating at 50 revolutions per second, for example, the position of the wheel will be the same in every frame and it will appear to not be moving. If the wheel is moving slightly faster, it will appear to be rotating slowly in a forward direction, but if it is rotating at, say, 48 revolutions per second, it will appear to be rotating backward. If there is a large difference in speed, however, the wheel will just be blurred.