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Demos: 7C-04 After Images

Students are asked to stare intently at an image for approximately thirty seconds. Then when they look at a white background, the image appears in its complementary colors.

Directions: Darken the room. Place the “ghost” transparency on the overhead. Ask the students to stare at a single spot for about thirty seconds. Then remove the transparency to produce a white screen. A reversed image of the ghost will appear on the screen.

Do the same for the US flag.

Suggestions for Presentation: Although the explanation for negative afterimages is somewhat complex, a simple model is that the cones of the eye become less and less responsive (“fatigued”) as light of the same color continues to be incident on them. For example, staring at a red spot will cause the “red” cones to become less responsive. This reduction in response lasts for several seconds. So if the red spot is removed and the student looks at a white screen, the red portion of the white light is not picked up very well, so the green and blue predominate. This produces a cyan afterimage.

Note: It has been stated in one publication on color that this is the reason hospital OR gowns are a bluish-green. The medial personnel constantly stare at red blood, so if they were to look up at a white lab coat, they would see cyan spots! (This hasn’t been verified as the reason!)

Applications: If a highway has its center stripes freshly painted and you are staring at the road as you drive, if you look away for a moment you will see blue stripes beside the yellow ones! Try other situations. Stare at a brightly colored flower and then look at a light background, for example.

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Last Updated: May 9, 2016 11:44 AM

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