rs « back to results for ""
Below is a cache of http://www.exploratorium.edu/cmp/exnet/exhibits/group3/benhams/media/benhams_s.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Benham's Disk 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.
3/4/02 2:56 PM
Exploratorium: Science Snacks: Benham's Disk
Page 1 of 3
http://www.exploratorium.edu/snacks/benhams_disk.html Posterboard or cardboard

or other suitable adhesive
Glue stick

(provided here)
Pattern disk
Access to a copy machine
A black marking pen
A rotator
You can use a turntable, variable speed electric drill, hand drill, portable electric mixer, or electric screw driver. Attach the disk with adhesive
Velcro, or if a drill with a chuck is used, a bolt can be used as a shaft, with two nuts to hold the disk. You can also reduce the size of the
disk on a copy machine, then mount it on the flat upper surface of a suitable toy top, or you can devise your own spinner for the disk. Try
spinning the mounted disk on a pencil point, or on a pushpin stuck into a pencil eraser.
Adult help
(15 minutes or less)
Copy the pattern disk in the drawing provided as a separate page,
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.
here
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.
3/4/02 2:56 PM
Exploratorium: Science Snacks: Benham's Disk
Page 2 of 3
http://www.exploratorium.edu/snacks/benhams_disk.html 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
, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).
Seeing the Light

Questions? Comments? Want to share your results?
Join the:

Like this snack? You
can order the
snackbook
online
for more perception
experiments!
"The
Cheshire Cat"

EXPLORATORIUM SNACK HOME PAGE
©
Exploratorium
3/4/02 2:56 PM
Exploratorium: Science Snacks: Benham's Disk
Page 3 of 3
http://www.exploratorium.edu/snacks/benhams_disk.html