Benham’s Disk Optical Illusion
Student Laboratory Kit
Materials Included In Kit
Benham’s Disk Sheets, laminated, 8
Dry-erase marker, black
Additional Materials Required
Colored pencils (optional)
Use scissors to cut out each Benham's disk from the laminated sheets. A total of 16 Benham's disks can be prepared.
Use caution when handling the pushpins. Follow normal classroom safety guidelines.
The Benham’s disks can be reused many times. Remove the dry erase markings from the disks with a damp paper towel.
- The larger the Benham’s disk, the higher the resolution will be between the observed colors of the rings. To demonstrate this feature, enlarge the Benham’s disk using a copy machine.
- The longer and thinner the line, the more brilliant the colors will be.
- Students can experiment with the thickness and style of the lines to discover any differences in the color brightness or color patterns.
- Use colored dry-erase markers and see if the same effect occurs.
- Instead of using a pushpin to rotate the disk, try turning it into a top. Cut a slit in the center of the laminated disk. Insert a coin or washer halfway into the slit. Twist the coin to spin the top. This may also be done using a toothpick rather than a coin or washer.
- Try using the Bracken’s Demonstration Spinner to rotate the disk—Flinn Scientific Catalog No. AP6202.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Planning and carrying out investigations
Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS4.B: Electromagnetic Radiation
HS-PS4.B: Electromagnetic Radiation
Cause and effect
Energy and matter
Answers to Questions
- What are the initial colors of the Benham’s disk?
The disk is half black and half white. The marked lines are black.
- Record your observations of the clockwise spinning disk and also when the disk stops. Draw a picture to illustrate any observed color patterns. Use colored pencils if desired.
The lines on the spinning disk appear to change into the colors of the rainbow. The colors disappear when the disk stops. Violet is on the outside and red is on the inside. It is more difficult to observe the red color than the violet color. Green is very distinguishable.
- Record your observations of the disk when it is spinning in the counterclockwise direction. Draw a picture to illustrate any observed color patterns. Use colored pencils if desired.
The lines on the spinning disk again appear to change into the colors of the rainbow. However, the color pattern is reversed. Now, violet is on the inside and red is on the outside. It is more difficult to observe the violet color now, compared to the red color. The green color is still very visible.
- Develop a hypothesis that might explain why the appearance of the disk changed when it is spinning clockwise and counterclockwise.
Student answers will vary. The color change could be the result of the difference in the speed of the lines on the disk. The inside lines will travel slower than the outside lines. Our eyes perceive the colors due to the different rotational speeds. The different locations of the lines determine the colors that are observed. The red arc is created by the first line in the rotating series. The violet arc is created by the last line in the rotating series. When the rotation is reversed the colors reverse order because the last line (violet) is now the first line and now appears red.
In 1894, the toymaker Charles E. Benham introduced his “Artificial Spectrum Top.” It later became known as the Benham’s top or Benham’s disk. When spun, the black and white top appeared to produce colored rings. The appearance of color is still a mystery even after more than 100 years, but it is believed to be at least partially the result of complex nerve responses in the cones of the retina. The cones of the retina allow individuals to see colors. Gustav Fechner (1801–1887), a German scientist and father of a branch of psychology known as psychophysics, was the first to describe the appearance of color from a spinning object in 1838. Psychophysics is the study of the relationship between stimulus intensity and a subjective experience (mental sensation) of the stimulus. The “pattern-induced flicker colors” that are produced by the Benham’s disk are known as subjective colors because the colors are perceived by our eyes due to the different response times of the cones of the retina. There are three color-sensitive cones on the retina—one for green, one for blue, and one for red. Each type of cone has different response and persistence times. For example, the “blue” cones have the slowest response times but they will continue to respond for the longest time after the stimulus has been removed.
When the spinning Benham’s disk is observed, alternating flashes of black and white stimulate the cones of the retina. White light has all three primary colors of light—red, green and blue. However, the brain only perceives white light when all the cones respond to the three primary colors equally. When the disk spins, each arc “flashes” at a different rate because each arc has a different amount of white space before and after. The different colors appear due to the location of the black arc on the disk. Lines that spin “into the black” with the least amount of white space between the black half-circle and the arc appear to be red in color. The middle arcs, with equal white space on each side, appear to be green. The arcs with the most white space between the arc and the black half-circle are blue. When the direction of the spinning disk is reversed, the arc that had the least amount of white space now has the most white space in the direction of the spin. This arc now appears blue. The middle arc still appears green, and the arc that previously appeared blue is now red.
http://www.michaelbach.de/ot/col_benham/ (Accessed June 2018)
http://serendip.brynmawr.edu/Mind/Consciousness.html (Accessed June 2018)