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If you stare at a waterfall for some time, and then stare at the rocks nearby, the rocks will appear to be moving upward. This illusion is known as the waterfall effect. Using the pattern provided here, you can create the waterfall effect—without getting wet.
For a simplified version of this activity, reduce the size of the disk on a copy machine. Mount the printed disk on the flat upper surface of a suitable toy top, spin the disk on the point of a pencil, or use a pushpin stuck into a pencil eraser. Whatever you can devise to get the disk spinning freely should be fine.
Rotate the disk slowly (1 or 2 seconds per revolution) and stare at its center for about 15 seconds. Now look at the palm of your hand. Notice that your palm seems to be turning. Your palm will turn in the opposite direction from the way the disk was turning.
Mechanisms in your eye and brain detect motion in various directions. For example, regions of your brain fire nerve impulses when your eye forms images that are rotating in a clockwise direction. Other regions respond to counterclockwise rotation. When something is stationary, both of these motion detectors still fire, but their firing rates are equal. The two signals balance each other out, and you see no motion.
As you stare at the spinning disk, the set of motion detectors that respond to its rotation adapts to the motion of the pattern. These motion detectors start out firing rapidly, and then slowly decrease their firing rate. When you look away from the rotating pattern and stare at a stationary object (such as your palm), the motion detectors that were already active fire less rapidly than the ones that have not been stimulated. As a result, you see motion in the opposite direction.
You also have sets of motion detectors that respond to upward and downward motion. Adaptation of these upward and downward motion detectors causes a similar effect (aptly known as the waterfall effect) that you notice when you stare intently at a waterfall and then look at a stationary cliff nearby. This effect was first noticed by Aristotle.
Try this effect on pictures in magazines or books. Stare at the spinning pattern disk, and then look right at the image. It will seem to pulse and squirm as if it were animated. This is especially fun to do if you have art books that show paintings (try a Picasso or something by Dali!).
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Attribution: Exploratorium Teacher Institute