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Exercise 4: Visual Illusion - Benham Discs

 

You can spend hours playing with this illusion and you can even make your own unique spinners too!

In 1894, toymaker Mr. C.E. Benham discovered that a spinning disk with a particular pattern of black and white marks could cause people to see colors. Mr. Benham called his disk an "Artificial Spectrum Top" and sold it through Messrs. Newton and Co. Benham's Top (or Benham's Disk) has puzzled scientists for over 100 years.

Here is how to make your own Benham Top:

1. Make the Spinner - **You must make at least one version below. You then need to test two different Benham disk graphics.**
Method #1

  1. Get an old compact disk (CD) and a penny.
  2. Clamp a penny with pliers.
  3. Heat the penny (I use a Bunsen burner). CAUTION: the penny must get very hot. Adult supervision is required.
  4. Insert the penny halfway into the center of the CD so that the penny is PERPENDICULAR to the CD. The hot penny will melt two slots on each side of the center CD hole. When the penny cools, it will stick in the middle of the CD. (See the picture below.)
  5. Twist the penny to spin the spinner.
Method #2

  1. Get the top of a plastic, round container (margarine containers work well) and a toothpick.
  2. Poke a very small hole in the middle of the plastic top. You may have to use a metal nail. The hole should be smaller than the width of your toothpick.
  3. Break or cut the toothpick in half. Insert the pointed end of the toothpick through the center hole in the plastic top. The toothpick should stick in the hole. (See the picture below)
  4. Twist the toothpick to spin the spinner.
Method #3

  1. Get some cardboard and a toothpick.
  2. Draw a circle (with a diameter of at least 4 inches) on the cardboard.
  3. Cut out the circle carefully. It is important that the circle is as round as possible.
  4. Poke a hole in the center of the circle with the toothpick.
  5. Insert the toothpick into the hole. The toothpick should stick out about half an inch. Break or cut the toothpick in half.
  6. Twist the toothpick to spin the spinner.

2. Get Benham disk graphics: Download and print out the disks.

Small Disks Disk #1 | Disk #2 | Disk #3 | Disk #4
Large Disks Disk #1 | Disk #2 | Disk #3 | Disk #4

3. Add a Disk to a Spinner
If you are using the CD spinner:
  1. Carefully cut out each disk.
  2. Cut a small slot in the center of the disk.
  3. Place the disk on the spinner by putting the slot over the penny on the spinner. The disk should lay flat on the top of the CD.
  4. SPIN THE SPINNER! Try other disks.
 If you are using the plastic top spinner or cardboard spinner
  1. Carefully cut out each disk.
  2. Poke a small hole in the center of the disk.
  3. Poke the toothpick through the disk.
  4. Put the toothpick (with the disk) through the plastic top or cardboard spinner. The disk should lay flat on the top of the spinner. You should use a little glue from a glue stick to keep it in place.
  5. SPIN THE SPINNER! Try other disks.

Special Notes

  1. The colors are seen best at slow speeds (between 3-5 rotations/second). Try different speeds!
  2. It is important that your spinner can spin at slow speeds. Therefore, make sure your spinner is centered properly. Make sure you place the penny or toothpick in the center of the spinner.
  3. Experiment!
    • Change the lighting conditions. Test it outside in sunlight; test it under different types of light (incandescent, fluorescent, etc.)
    • Change the pattern. Make you own pattern using the blank disk. Use a black marker to draw a pattern or series of arcs on the white side of the disk.
    • Change the color of the disk. What happens if you use a blue disk?
    • Spin the spinner clockwise and counter-clockwise.
 

 

What's Happening? What Causes the Colors?

The retina of the eye is composed of two types of receptors sensitive to light: cones and rods. Cones are important for color vision and for seeing in bright light. There are three types of cones, each of which is most sensitive to a particular wavelength of light. Rods are important for seeing in low light.

It is possible that the colors seen in spinning Benham disks are the result of changes that occur in the retina and other parts of the visual system. For example, the spinning disks may activate neighboring areas of the retina differently. In other words, the black and white areas of the disk stimulate different parts of the retina. This alternating response may cause some type of interaction within the nervous system that generates colors.

Another theory is that different types of cones take different times to respond and that they stay activated for different amounts of time. Therefore, when you spin the disk, the white color activates all three types of cones, but then the black deactivates them. The activation/deactivation sequence causes an imbalance because the different types of cones take different times to respond and stay on for different times. This imbalance in information going to the brain results in colors.

Neither of these theories explains the colors of Benham's disk completely and the reason behind the illusion remains unsolved.

 

*adapted from - Washington University Senses Lab

 

 

  

Comments or suggestions feel free to let me know -- Gary Glaser -- gwglaser@genesee.edu