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Retinal Fatigue (Sabrina Loesh)


Author

Sabrina Loesh

Principle(s) Illustrated

  1. Parts of the eye including retina and photoreceptors.
  2. Properties of light: Additive color and subtractive color.

Standards

  • 7.5.g - Students know how to relate the structures of the eye and ear to their functions.
  • 7.6.b - Students that for an object to be seen, light emitted by it or scattered from it must be detected by the eye.
  • 7.6.e - Students know that white light is a mixture of many wavelengths (colors) and that retinal cells react differently to different wavelengths.

Questioning Script

Materials

  • Flag in complementary colors
  • Color rectangle image
  • Laptop with projector
  • or OH transparency, overhead pens and OH Projector

Procedure

  1. Project the flag image or create an overhead transparency using overhead pens. 
  2. Instruct your students to cover one eye, and stare intensely at the middle of the flag with the other eye.
  3. After 1 minute, remove the transparency, and instruct students to keep their eyes focused on the same point on the white screen.
  4. They should soon see the American flag, correctly colored in red, white and blue!
  5. Instruct students to draw a red circle on a white paper using a red marker.
  6. They should cover one eye, place the circle in the bright light, and stare at it. 
  7. At the end of one minute instruct them to quickly refocus on a well-lit sheet of paper.

Prior knowledge & experience:

Most students will have encountered the following scenario:

When you are photographed with a camera equipped with an electronic flash, you may notice a dark after-image of the strobe long after the flash of light has disappeared. 

Root question:

  1. What colors do the flags appear after staring at them and then refocusing your eyes on the blank white paper? (red, white, blue)
  2. Identify the color of the after-image when you stare at the dot? (magenta)
  3. Why do the colors appear to change when you refocus on the white paper? (photoreceptive cells are overstimulated and fatigue causing the image to appear in complementary colors)

Target response:

Staring at the flag, rectangular image or dot fatigues the photoreceptive cells in your retina so they are temporarily unresponsive to light. As a result, when you look at a white paper or screen after staring at the image, you will see the object in complementary colors. Light reflected from white paper normally stimulates the red, green and blue cones of the retina, but if you have first fatigued the red cones by staring at a red object, these cells temporarily will not respond to red light. As a result, only green and blue cones in the region of the retina are stimulated, causing the image to appear cyan, the complementary color of red.

Common Misconceptions:

Under normal conditions, we don't see colors where none exist, so when students observe colors when they stare at a sheet of white paper, their curiosity is immediately aroused. You can use this discrepant event as an introduction to the importance of observation in the scientific method or to introduce students to vision and retinal physiology.

Photographs and Movies


References

Flag Image

Color Rectangle Image

Subtractive Color Gizmo

Additive Color Gizmo


Herr, N. (2008) The Sourcebook for Teaching Science: Strategies, Activities, and Instructional Resources. San Francisco, CA: Jossey-Bass.
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