Afterimages are optical illusions that occur after looking
away from a direct gaze at an image. This is closely related to the phenomenon
called the persistence of vision, which is used in animation and cinema. One of
the most common afterimages is the bright glow that seems to float before one's
eyes after staring at a light bulb or a headlight for a few seconds.
Afterimages are caused when the eye's photoreceptors, primarily those known as cone cells, "tire" from the over stimulation and lose sensitivity. Normally the eye deals with this problem by rapidly moving the eye small amounts, the motion later being "filtered out" so it is not noticeable. However if the color image is large enough that the small movements are not enough to change the color under one area of the retina, those cones will eventually tire and stop responding. The rod cells can also be affected by this.
When the eyes are then diverted to a blank space, the tired photoreceptors send out little signal and those colors remain muted. However, the surrounding cones that were not being used are still "fresh", and send out a strong signal. The signal is exactly the same as if looking at the opposite color, which is how the brain interprets it. For example, a lime green image will produce a purple afterimage, because the lime green color does not tire out the purple photoreceptors.
To see an afterimage of the picture below, stare at it for 45 seconds, then immediately look at a blank wall or piece of white paper to see the afterimage. In the afterimage, an image of Jesus will appear - blink a couple of times if you don't see the image.