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Canadian Flag After-Image

In this demonstration, colours appear before the students' eyes as they learn about cell fatigue and afterimages.

Our central nervous system stops responding to information from our senses that is constantly repeated. You're not aware of the feeling of your clothes on your skin (until I mentioned it) because that feeling doesn't change. This is because you have adapted to the stimulus (the feeling of your clothes). This is useful so that you will notice a sudden and threatening change in your environment, such as a tiger attack. It would be unfortunate if your brain was busy paying attention to your clothes touching your skin and didn't notice a tiger!

Staring at the green colour in the flag tires out the cone cells in your retina that respond to green. Since the brain gets used to looking at the green in the flag, the cone cells send fewer and fewer signals saying "Green stuff here!" to the brain. While you're looking at the flag, your eyes are technically less sensitive to green since it is "old news" for the brain.

When you switch over to a white surface, your eyes don't register the green light and you see its complementary color red for a few moments. Remember that what our brain processes as "white" is actually the combination of all colours combined. It takes a few moments for the cones to start firing green signals to the brain again, which is why we see the complementary colour red as an "afterimage"

In the same way, if you stare at a red color for 30 seconds or more, the cells in your retina that respond to red will fatigue more and fire less. When you switch over to a white surface, your eyes subtract the red and you see its complementary color green.

Teacher's Note: This image MUST be printed in colour.

Objectives

  • Understand the role of cone cells in the eye.

Materials

  • Per Class or Group:
    green Canadian flag template
    , printed on either an overhead sheet or on a large sheet of paper
    overhead projector (optional)
    large white board, wall or card

Key Questions

  • What do you see when you stare at the white wall?
  • Why don’t we see a green maple leaf, like the original flag we looked at?

What To Do

  1. Hold up the peculiar green version of the Canadian flag.
  2. Instruct students to stare at the image for 30 seconds (You can have them hum the National Anthem while they stare).
  3. Take the green flag away, and tell the students to stare at a white surface.

Extensions

  • Try making your own after-image. Use different colours and record the complementary colour that appears.

Other Resources

Science World Resources | Drapeau illusoire | French version of this resource