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It is the weekend and you are sitting in a dark movie theatre with popcorn in one hand and a box of chocolate covered raisons in the other. You sit there for over an hour staring at the image that is projected on the large screen in front of you.

But did you know that for part of this movie you are actually sitting in a dark theatre with no image on the screen? There is a projector that is located in the back of the theatre that projects the movie that you see onto the screen. There is also a very powerful light bulb in the projector that shines on the film as it passes through the projector. When the light passes through the film the image on the film is projected onto the screen.

The actual movie film looks just like regular film that comes from your camera. If you could stretch out a section of film you would see a series of several still images. But how does this whole process work? If movie film is a series of still images why do we see a moving image on the screen? And how can we be in a dark theatre with no image on the screen without noticing? You will be happy to know that all of these questions can be answered by two very simple phenomenons- Persistence of Vision and the Phi Phenomenon.

Persistence of Vision is the result of the tendency of the eye’s retina (the part of the eye that is sensitive to light) to retain an image for about 1/12 of a second. Persistence of Vision was first noted by Peter Mark Roget in 1820. If you swing a lighted stick around in a circular motion you will see the illusion of a “circle” of light. This is because of the Persistence of Vision phenomenon. Basically, if your retina sees successive images at a rate faster then about 12 per second, your eye will blend them together. This is because when you see the next image your retina is still “seeing” the previous image. This is the same reason why the world does not go suddenly pitch black every time you blink your eyes. Pretty amazing, huh?

Well, the movies that you watch in a movie theatre take advantage of this human phenomenon. Movie film moves through a projector at 24 frames per second. That means that 24 still images (just like the images from a picture from your camera) move through the projector every second. These images move intermittently through the projector stopping for a fraction of a second on each image. When the film is stopped, the image shines on the screen. Then, the light from the projector is blocked by a shutter for a fraction of a second while the film jumps to the next image. We do not “see” the temporary darkness because we are still “seeing” the previous image. So, the time that you are sitting in a dark theatre is “ignored” by the brain. This sequence of moving film is repeated at 24 frames per second for the entire movie.

The Phi Phenomenon is simply the tendency for our brains to connect these images and give them movement. An example of this phenomenon is a series of flashing lights that appear to be moving along a path (such as blinking Christmas lights). When the individual lights blink in a consecutive series, one after the other, it appears as if one light is moving along the path when actually nothing is moving at all. Our brains can do some interesting things, huh! You will now never “look” at movies in the same way.