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Benham’s Disk

Students make a contraption that appears to produce colours from black and white.

Benham's Disk was invented by a nineteenth-century toymaker who noticed colours in a black-and-white pattern he had mounted on a spinning top.

Why do we see colours? 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 stimulus, 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 and keep responding the longest.

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. Your eyes and brain see the colour 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 partially explains why you see colours.

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

Objectives

  • Understand the role of cone cells in the eye.

Materials

  • Per student:
    Benham’s disk template (printed on cardstock)
    a pin
    a pencil with an eraser tip
    scissors

Key Questions

  • What do you see? What changes when you start spinning the disk? Where do you think the colours come from? Why are there different colours in different places on the disk?

What To Do

  1. Cut out the Benham’s disk template.
  2. Poke a pin through the middle of the circle and then fix the disk to the pencil by poking the pin into the eraser. 
  3. Spin the disk and observe what happens

Teacher Tip:  The key to this illusion is to keep the disk spinning fast, continuously, and smoothly.

Extensions

  • Try spinning the disk in the opposite direction. Try spinning the disk in dim light.

Other Resources

Science World Resources | Full Unit | Illusions
Science World at TELUS World of Science | Puzzles and Illusions