TOPIC:

ID Code: O3-04
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 right half covered by a green and the left half by a magenta filter, the second with the right half covered by a green filter but with the left half open so it is white; both have 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 continuing to stare at the black dot in the center. On the second slide the green half on the right is the same as the first slide, but the magenta left side of the first slide is replaced on the second slide by a white field. The white field viewed on the left side of the second slide is actually greener than the actual green field at the right.

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 right side, but saturates your red and blue (magenta) receptors on the left side. When the second slide comes up, the green receptors on the right remain saturated (even more), causing the color to be washed out. However, the magenta side has saturated your red and blue receptors, but left your green receptors totally unused. Therefore, 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.

To perform the experiment using your video monitor, click on the photograph above to get the first slide, then click on that slide when you have stared at it for 20-30 seconds to saturate your eyes. Prepare your computer to do the transitions rapidly by cycling through the photographs to place them in your cache memory or use the PowerPoint program linked below. However, this only works if your monitor is at the proper pixel size.

Availability: Available

Some links on a related color perception topic that made the news recently:
www.washingtonpost.com/news/morning-mix/...at-divided-a-nation/
xkcd.com/1492/
en.wikipedia.org/wiki/Checker_shadow_illusion
www.mantlelabs.com/illusion/

See the Physics Question of the Week for 1/12/2015 for an example of how this demonstration can be used in class interaction and discussion: lecdem.physics.umd.edu/question-of-the-week-archive.html