system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when you look at something very bright it leaves a “mark” that floats in your field of vision when your eyes are opened or closed?
gpt
At the back of your eyes, in the cells called “rods” and “cones,” there are little bits of pigment. The pigment absorbs photons—pieces of light—of certain wavelengths, depending on what color that cell handles.

When a photon hits a molecule of pigment, the molecule of pigment chemically splits and gives off an electrical impulse. This impulse is picked up by the sensory neurons and related to the brain.

The problem is that now the pigment is broken. If something very bright is in your field of view, all the pigment in that spot on your retina will break. You’ll run out of gas.

That means when you look away, even the normal impulses that would trigger signals in that color don’t work from that spot of the retina. We interpret that as a spot or mark in our vision (or also that kind of illusion where you look at a funky-colored image for a while then look at a white blank space and you see the color it was supposed to be as an afterimage).

Eventually, the pigment molecules reassemble themselves and your vision goes back to normal (unless you damaged your retina), but this takes some time—a few minutes, usually.