Question: Why do imprints of bright things get left in our eyes?

Michael Kelly answered on 20 Jun 2014:

There are two parts to this. Firstly our eyes/brain adapt to things that are constant and when these things are removed then our brain has to readjust. This is quite quick taking about 15~30sec. The second mechanism is a bit more complicated. When the sense cells absorb light the pigment becomes activated this changes its shape and triggers a cascade of chemical events that result in an electrical signal. However, the activated pigment keeps triggering signals until it is deactivated. This is what makes the after image after a bright flash, and can take many minutes to disappear.