Afterimages occur because photochemical activity in the retina continues even when the eyes are no longer experiencing the original stimulus.

Why do I keep seeing afterimages?

What causes afterimages? Negative afterimages occur when the rods and cones, which are part of the retina, are overstimulated and become desensitized. This desensitization is strongest for cells viewing the brightest part of the image, but is weakest for those viewing the darkest.

What is an afterimage?

A flash of light prints a lingering image in your eye. After looking at something bright, such as a lamp or a camera flash, you may continue to see an image of that object when you look away. This lingering visual impression is called an afterimage.