The Phi Phenomenon: Definition & Example

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  • 0:00 What Is the Phi Phenomenon?
  • 1:25 Beta Movement
  • 2:09 How Does the Phi…
  • 3:04 Lesson Summary
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Lesson Transcript
Instructor: Emily Cummins
Ever heard the expression 'Your eyes are playing tricks on you?' Well, in some cases they might be! In this lesson, we'll talk about the perceptual illusion known as the phi phenomenon. Then, you'll be able to test your new knowledge with a quiz.

What Is the Phi Phenomenon?

Remember those books of still images, known as flip books that you had as a child? When you rapidly flipped the pages, it appeared as if the still images were actually moving. This is a kind of optical illusion of sorts, and it's a good example of the phi phenomenon, a psychological term that describes the optical illusion of seeing a series of still images as moving.

Here's the most common example of this: When you go to a movie theater, you see a rapid succession of still frames that make up the whole movie. But it doesn't look to your eyes like the film reel is flipping through a bunch of frames. It looks like one complete image.

The discovery of the phi phenomenon is attributed to Max Wertheimer, a German psychologist who studied sensation and perception. To demonstrate how the phi phenomenon works, researchers projected a line on the left side of a projector, and then a line on the right side of the projector. They did this in rapid succession. When asked what they saw, people observing indicated that they saw one line moving back and forth.

Basically, Wertheimer argued our brain filled in the space between the two lines, making it seem like the line on the left was moving to the right, instead of the two stationary lines that were shown. The faster these two lines are shown in succession, the more our brain tricks our eyes into thinking it's one line moving back and forth.

Beta Movement

The phi phenomenon is quite similar to another perceptual illusion described by Wertheimer, called beta movement, an allusion in which your brain combines two images or more, which you then you perceive them as moving.

So, for example, say you're looking at a computer screen and see several dots arranged in a circle. Then, each dot in the circle blinks in a clockwise fashion. What is really happening is that each circle is lighting up, but what it looks like to your eyes is that one circle is jumping around in a clockwise motion.

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