An afterimage or ghost image is an optical illusion in which you continue to see the image briefly even after the exposure to the actual image has stopped. Here is an activity to prove this point.
Stare at the yellow leaf in the middle of the square for about 30 seconds. Then move your gaze to the white space on the right, and see what happens. Did the colours really reverse themselves? This is an example of an "afterimage".
The retina contains rods and cones. Cones are sensitive to colour changes. When you stare at a particular colour for too long, these colour receptors get tired. Then, when you look away, the cones for that colour relax and for a moment don't work as well. Therefore, you see everything but the original colour. An afterimage appears when the eyes see something that isn't there.
“Sight Vision.” Neuroscience for Kids. n.d. Web. 23 March 2013. <http://faculty.washington.edu/chudler/chvision.html>.
Deb. "Kid Questions: Why do we see the lights after they are switched off?" Science@home. 5 Dec 2011. Web. 18 Apr. 2013. <http://science-at-home.org/afterimage/>.