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.