If you look at a brightly lit scene in just the right way and then close and cover your eyes you will see a positive afterimage of the scene.
Prepare a window by placing a fluorescent dot in the middle of the glass. Note that if the window is made of panes of glass separated by a wood frame you can skip placing the dot and just use one of the places where the wood makes a + instead.
To Do and Notice
Close and cover your eyes, for example cover them with your hands, keeping them as dark as possible for at least 15 seconds.
Open your eyes and stare at the fluorescent dot for 3 seconds. Don't move your eyes while you look.
Close and cover your eyes again.
What do you see?
Many people see a positive afterimage of the scene. With time the image changes intensity and color.
After a while remove your hands but keep your eyes covered to allow some diffuse light to leak through your eyelids. Then cover your eyes with your hands again.
Open your eyes and look at a white piece of paper, you may see a negative afterimage.
What's going on?
When you cover your eyes they begin to adapt to the dark. They become more sensitive to light.
When your make an image of a scene on your retina, the cones are stimulated to produce nerve impulses. These nerve impulses continue after the light has been turned off. The continued production of nerve impulses after your eyes are closed and covered produces a positive afterimage.
A positive afterimage is bright where the image is bright and dark where the image is dark.
If you look at a bright white piece of paper you may see a negative afterimage. The cones in the bright portion of the original scene have adapted to bright light and so respond less to a uniform field of white light such as a piece of paper. The bright regions in the original scene thus appear dark in the negative afterimage.
Try dark adapting your eyes for longer periods of time. With your eyes closed try "looking" at a bright light and observe what happens to the afterimage. Try this experiment with one eye then expose the other eye to bright light while it is closed.
Scientific Explorations with Paul Doherty
3 October 2003