PURPOSE: Demonstrate negative color afterimage due to saturation.

DESCRIPTION: Here's the question: What is greener than green? To find the answer, you must carry out the following experiment. Two slides are prepared, the first with the left half covered by a green and the right half by a magenta filter, the second with the left half covered by a green filter but with the right half open (clear plastic); both having a black dot in the center to stare at.

Everyone stares at the dot in the middle of the first slide for about 20 to 30 seconds. That slide is then quickly replaced by the second slide, with everyone again staring at the black dot in the center. On the second slide the green half on the left is the same as the first slide, but the magenta right side of the first slide is replaced on the second slide by a white field from the projector. The green field seen on the right side of the second slide is actually greener than the green field at the left.

Why do you see green on the right side of the second slide, and why is it greener than the actual green side? Staring at the first slide saturates your green receptors on the left side, but saturates your red and blue (magenta) receptors on the right side. When the second slide comes up, the green receptors on the left are saturated even more, causing the color to be washed out. However, the magenta slide has saturated your red and blue receptors, but left your green receptors totally unused. When the white light is seen, red and blue are washed out but the green receptors are strongly excited, leading to a beautiful green field.



EQUIPMENT: Two slides, as shown, with slide projector.


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