Persistence of Vision
Human vision does not operate like a camera, taking a sequence of images and passing them to the brain to be processed. The brain attempts to put a construct on what is passed to it, so that the world makes sense. The impression an image makes on the brain persists for a tiny fraction of a second, so that if, before the impression of that image disappears, another image appears, slightly different, an impression can of continuity and movement can be made. Experiment suggest that presenting the brain with 15 images per second can maintain an impression of continuity and movement. Any less than this, an impression of flicker appears, which can cause eyestrain.