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In the phi phenomenon, a sequence of images causes us a movement sensation

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Example of the beta movement, often confused with the phi phenomenon

The phi phenomenon is the optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession. The phenomenom was defined by Max Wertheimer in the Gestalt psychology in 1912 and along with persistence of vision formed a part of the base of the theory of cinema, applied by Hugo Münsterberg in 1916.

Persistence of vision

Persistence of vision refers to the phenomenon in which the human perception of the decay of a visual stimulus is slower than the true decay of that stimulus. An image will stay on one's eye for a brief amount of time after its cause has, in reality, disappeared.

The phenomenon of the persistence of vision is popularly taught as the reason that humans perceive motion in such things as zoetropes and classically projected films, but it is in reality not connected with motion perception. It is merely the reason that we do not see the black frames that come between each "real" frame while watching a movie. The true reason for motion perception is the phi phenomenon.

Examples of use of the phi phenomenon

Cinema and other popular forms of animation are, of course, the best example of this phenomenon at work. However, some of its predecessors are as follows:


The phenakistoscope was an early animation device. It preceded the zoetrope.


Meaning "wheel of life," the zoetrope is a device that produces the illusion of motion by presenting static pictures in quick succession. It accomplishes this by way of a spinning drum with slits in the top through which one watches the animation, which has usually been drawn on a strip of paper that sits at the bottom of the drum.

Experiment of the phi phenomenon

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.

At a certain combinations of spacing and timing of the two images, a viewer will report a sensation of motion in the space between and around the two lines. In these cases, the line that seems to move is actually a figure that first appears in the right of the screen and then in the left.

The phi phenomenon is not beta movement

Although both cause sensation of movement, the phi phenomenon can be considered to be an apparent movement caused by luminous impulses in sequence, whereas beta movement is an apparent movement caused by luminous stationary impulses.[1]

See also


  1. Phi is not Beta – An explanation of the difference between the beta and phi phenomena, with online demonstrations of both, and a stronger version of phi than that found by Wertheimer

External links

  • The Myth of Persistence of Vision Revisited – A detailed explanation of how the perception of motion in film and video differs from the simplest notions of "persistence of vision", with mention of the erroneous use of phi as a revised explanation.
  • Phi phenomenon activity – Application that lets us change some parameters to experiment with the phi phenomenon.
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