Phi phenomenon is an optical illusion in which the rapid appearance and disappearance of two stationary objects such as flashing lights are perceived as the movement back and forth of a single object. The phi phenomenon is a perceptual illusion described by Max Wertheimer in 1912, in which a disembodied perception of motion is produced by a succession of still images.
The classic phi phenomenon experiment involves a viewer or audience watching a screen, upon which the experimenter projects two images in succession. The first image depicts a line on the left side of the frame. The second image depicts a line on the right side of the frame. The images may be shown quickly, in rapid succession, or each frame may be given several seconds of viewing time. Once both images have been projected, the experimenter asks the viewer or audience to describe what they saw.