Afterimages

An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased. One of the most common afterimages is the bright glow that seems to float before one's eyes after looking into a light source for a few seconds. The phenomenon of afterimages may be closely related to persistence of vision, which allows a rapid series of pictures to portray motion, which is the basis of animation and cinema.

If the viewer stares at this image for 20-60 seconds and stares at a white object a negative afterimage will appear (in this case being cyan on magenta). This can also be achieved by the viewer closing his/her eyes and tilting their head up.Closing the eye can help achieve a better sense of the color in its own aspect.

Afterimages come in two forms, negative (inverted) and positive (retaining original color). The process behind positive afterimages is unknown, though thought to be related to neural adaptation. On the other hand, negative afterimages are a retinal phenomenon and are well understood.

Here is a video that shows examples:

http://www.youtube.com/watch?v=Z4oURqnIJGI