Materials Included In Kit

Additional Materials Required

Prelab Preparation

Use scissors to cut out each Benham's disk from the laminated sheets. A total of 16 Benham's disks can be prepared.

Safety Precautions

Use caution when handling the pushpins. Follow normal classroom safety guidelines.

Disposal

The Benham’s disks can be reused many times. Remove the dry erase markings from the disks with a damp paper towel.

Teacher Tips

• 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.

Answers to Questions

1. What are the initial colors of the Benham’s disk?

   The disk is half black and half white. The marked lines are black.

2. 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.

   {12772_Answers_Figure_2}
3. 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.

   {12772_Answers_Figure_3}
4. 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.

Discussion

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.

Teacher Handouts

12772_Teacher1.pdf

References

http://www.michaelbach.de/ot/col_benham/ (Accessed June 2018)

http://serendip.brynmawr.edu/Mind/Consciousness.html (Accessed June 2018)

Introduction

Display the colors of the rainbow on a rotating black-and-white disk.

Concepts

• Colors of the visible spectrum

• Persistence of vision
• Optical illusions

Materials

Safety Precautions

Use caution when handling the pushpin.

Procedure

1. Obtain a blank, laminated Benham’s disk, a pushpin and a black dry erase marker.
2. Use the black dry erase marker to draw arcs on the white portion of the disk similar to those shown in Figure 1.
   {12772_Procedure_Figure_1}
3. Poke a hole in the center of the Benham’s disk using a pushpin. Rotate the pin in the hole to widen the hole slightly. This will allow the disk to spin freely on the pushpin.
4. Record the initial colors on the Benham’s disk on the Student Worksheet.
5. Place the disk onto the pushpin with the black semi-circle facing up.
6. Carefully, yet quickly, rotate the disk clockwise on the pushpin. Continue rotating the disk and observe the pattern created by the spinning disk. Are any colors observed? When the disk stops rotating, are there any changes from the original appearance? Record all observations on the Student Worksheet.
7. Rotate the disk counterclockwise on the pushpin. Continue to rotate the disk and observe the pattern created by the spinning disk. Are any colors observed? Have the locations of the colors changed? Record all observations on the Student Worksheet.
8. Review your observations to develop a hypothesis explaining the observed changes. Enter this hypothesis in the Student Worksheet.

Student Worksheet PDF

12772_Student1.pdf