Observations and results
In the afterimage did you see that the top right part of the circle was cyan colored, the bottom part was purple-magenta and the top left part was yellow?
If you stare at a red object and immediately look at a white area afterward, you will see an afterimage that is the same size and shape, but it is blue-green, or cyan, in color. This is because your eyes use the red, green and blue cone cells to perceive white light, but because the red cone cells are fatigued, you do not see red. You are temporarily left seeing with only your green and blue cone cells. This is the same process that happened to your eyes in this activity, and it is why the color of each piece of the circle in the afterimage is a mixture of two of the three additive primary colors (red, blue and green), specifically the two that were not in the corresponding piece of the original image. Mixing two of the three primary colors results in the following secondary colors: red and green appears as yellow, red and blue becomes purple (including magenta), and green and blue turns to cyan.
More to explore
Afterimage, from Dresden University of Technology
2013 Best Illusion of the Year Contest, from Neural Correlate Society
The Eye, from George Mather, University of Sussex
Are Your Eyes Playing Tricks on You? Discover the Science Behind Afterimages!, from Science Buddies
This activity brought to you in partnership with Science Buddies