Persistence of Vision Demonstration

by John C. Lahr (4/19/97)

At the Exploratorium in San Francisco there is a flickering light hanging high above the main entrance. The light is composed of a vertical band of lights which are rapidly turned on and off in a repeating sequence. From the upper deck one can "read" this sign by looking first to the left of it and then to the right of it rapidly. The word EYE appears. The letters are symmetric with respect to a vertical line through the center of the Y, so that the sign reads EYE from either direction. The word EYE is actually "painted" on one's retina and appears to remain there for a split second due to persistence of vision.

It is relatively easy to set up a home demonstration of persistence of vision. Here's what is needed:
bulletrecord player (33 or 45 rpm)
bullettwo unneeded 5.5 inch floppy disks
bulletwhite correction fluid (white out)
bulletscotch tape
bulletscissors

Cut off one edge of the plastic outer sleeve of the two floppy disks and remove the disks from the inside.

WORD DISK: On one disk, the "word disk", write some short words with the correction fluid around the edge. Keep the top of the words at least 4 mm from the edge of the floppy disk and make the letters from 1 to 2 cm high. Draw a white line above each word along the outer edge of the floppy disk as indicated in this figure.

SLOT DISK: Trim 2 mm from the edge of the second "slot disk", so that it's diameter is reduced by about 4 mm. Cut a 2 mm-wide 3-cm deep radial slot in this floppy disk. The smaller floppy disk should cover the words but not the lines above the words of the "word disk", and the slot should extend as deep as the lower edge of the words.

Cut four 6 cm-long pieces of scotch tape, placing two face to face across the center of each disk. Cut or punch a hole in the "word disk" large enough that is will rest on the turntable and turn like a record.

Cut or punch a smaller hole on the "slot disk." Slide it down the spindle of the record player to a position just above the "word disk." The hole in the "slot disk" should fit tightly around the spindle so that it will remain stationary while "word disk" turns below.

Orient the slot so that it points away from you. As the "word disk" turns, lettering will be visible through the slot as the words pass below it. If one stares at the slot, however, it will not be possible to read the words. To read the words, focus on the line above one of the words and follow it around and around with your eyes as the word disk turns. When the line passes above the slot, the word below will momentarily become visible. The letters, in a sense, are "painted" on the retina but appear to be visible all at once due to persistence of vision.


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