Optical illusion

persistence of vision

 

The moving picture above is composed of four frames. These frames are shown in quick succession which gives the illusion of movement.

The reason for this is that the retina holds the image for a fraction of a second even after the object has been removed and there is nothing to see. This is called persistence of vision. If a second picture (slightly altered) replaces the first the two images merge appearing to have changed slightly. If the images are replaced rapidly it looks as if the pictures are moving.