Just about everyone has experienced them. Afterimages occur after a person has stared fixedly for several seconds at an image, a bright light or an especially vivid pattern. When the person directs his or her gaze to another area, a ghost image of the object, in a reverse or complimentary color, appears superimposed on the new background. The afterimage may have nearly the identical clarity of detail as the original object or light, or appear somewhat blurred.
The afterimage color often reproduces as a reversal of color from white to black or black to white. However, these ghost pictures also can assume different rainbow hues, depending on the length of staring time and the color of the object. For instance, the original image, if yellow should reproduce as blue; if red, as cyan; if green, as magenta. It may require a bit of practice to identify these colors in the afterimage.
If one purposely induces afterimages, a phenomenon peculiar to this type of optical illusion takes place. An afterimage will appear larger when cast upon a distant than on a nearby background, as on a wall across the room as opposed to a sheet of paper held close to the eyes. Similarly, the moon seems markedly larger when viewed near the horizon, perceived by the mind as being far away, than it does when studied directly overhead amongst the "closer" stars. This seeming differential in size occurs even though the object or pattern viewed has the same dimension regardless of its spatial relationship to the person staring at it.
A darkened room provides an excellent setting in which to experiment with afterimages for informal analysis. Start by stretching a white sheet of paper over the lens of a flashlight. Affix the paper to the flashlight with tape or a strong rubber band. Use a marking pen to blacken all of the paper's surface except a small circle or square in the center. Turning on the flashlight permits a strong but diffused light to shine through the paper.
With overhead and other lights in the room switched off, aim the flashlight to direct its beam into your eyes. Stare at the square or circle of illumination for several seconds, up to half a minute or so, then douse the flashlight. A comparison of the color and/or size of the afterimage now can be made by focusing on a bare wall of the room and then on something nearer at hand.
Afterimages, then, fall into a broad category of perfectly natural phenomenon and have no relationship to supposed supernatural events. So the next time you run through a dozen or so games of FreeCell on your computer, one after the other, do not become alarmed if ghostly images of playing cards appear superimposed on the backdrops of your surroundings.