Benham's Disk
A rotating black-and-white disk produces the illusion of color.

When you rotate this black-and-white pattern at the right speed, the pattern appears to contain colored rings. You see color because the different color receptors in your eyes respond at different rates.

(15 minutes or less)

Copy the pattern disk in the drawing provided as a separate page, here and mount it on a cardboard backing with the adhesive. If your copier does not make good solid blacks, fill in the black areas with a black marking pen. You can reduce or enlarge the pattern disk if you like.

Attach the mounted disk to a rotator.

(15 minutes or more)

Spin the disk under bright incandescent light or sunlight. (Fluorescent light will work, but there is a strobing effect that gives the disk a pulsating appearance and makes it harder to look at.)

Notice the colored bands that appear on the disk. Look at the order the colors are in. What color do you see at the center? What about the next few bands?

Reverse the direction of rotation and compare the order of colors again, from the center of the disk to the rim.

Try varying the speed of rotation and the size of the pattern, and compare the results with your initial observations.

Different people see different intensities of colors on this spinning disk. Just why people see color here is not fully understood, but the illusion involves color vision cells in your eyes called cones.

There are three types of cones. One is most sensitive to red light, one to green light, and one to blue light. Each type of cone has a different latency time, the time it takes to respond to a color, and a different persistence of response time, the time it keeps responding after the stimulus has been removed. Blue cones, for example, are the slowest to respond (have the longest latency time), and keep responding the longest (have the longest persistence time).

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. When a white flash goes by, all three types of cones respond. But your eyes and brain see the color white only when all three types of cones are responding equally. The fact that some types of cones respond more quickly than others -- and that some types of cones keep responding longer than others -- leads to an imbalance that partly explains why you see colors.

The colors vary across the disk because at different radial positions on the disk the black arcs have different lengths, so that the flashing rate they produce on the retina is also different.

The explanation of the colors produced by Benham's disk is more complicated than the simple explanation outlined above. For example, the short black arcs that are on all Benham's disks must also be thin, or no colors will appear.

Benham's disk was invented by a nineteenth-century toymaker who noticed colors in a black-and-white pattern he had mounted on a top. Toy spinning tops with Benham's disks are still available in the Exploratorium Store and in toy stores.

The three different color sensors in a color television camera also have different latency and persistence times. When a color television camera sweeps across a bright white light in its field of view, it often produces a colored streak across the television screen.

When your eye scans a black-and-white pattern containing fine detail, you will sometimes see subtle colors. For more information, see the book Seeing the Light, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).