The phi phenomenon is the optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession. The phenomenon 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.

Beta movement is often confused with the phi phenomenon but they are different. The phi phenomenon is the apparent motion caused by a changing static image, as in a motion picture. The beta phenomenon is the apparent motion between different light sources that are periodically switched on, as in chase lighting. In phi, there are different images or lights in a single place, whereas in beta the images or lights are in different locations. In both cases the images or lights are turned on and off rapidly to produce the effect.

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