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Squirming Palm

Science Snack
Squirming Palm
This visual illusion makes the palm of your hand appear to squirm and twist.
Squirming Palm
This visual illusion makes the palm of your hand appear to squirm and twist.

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.

Tools and Materials
  • Pattern disk (provided in the Assembly section below)
  • White card stock (alternatively, you can print the pattern on plain white paper and then mount it on posterboard)
  • Scissors
  • A black marking pen
  • Variable-speed electric drill (works well because it can be reversed)
  • Machine screw (e.g., 1024, 3 in or 4 in), 2 washers (e.g., SAE 10), and a wingnut (e.g., 10-24) to fit the machine screw, or you can use double-sided tape to attach your disk to your rotator
Assembly
  1. Print a copy of the pattern disk below (click to enlarge to full size) on either white card stock or plain white paper. The diameter of the printed disk should be about 6.5 inches (16.5 cm).
  2. Cut out the disk and, if your printer does not reproduce good solid blacks, fill in the black spots with a black marking pen. If using plain paper instead of card stock, mount your printed pattern disk on posterboard.
  3. Poke a hole in the center of the pattern disk and mount it on the machine screw so that it is between the two washers and held in place by the wingnut. Insert the screw into the drill and tighten the chuck to hold it in place.

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.

To Do and Notice

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.

What’s Going On?

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.

Going Further

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!).