your vision works like this: there is a patch on the back of your eyeball, and two types of detectors work here: rods and cones. these two types of cells work together in tandem to produce the vision you currently see. as light hits these cells, it basically lights up those cells. one pixel is probably lighting up a couple thousand cells together. the longer you stare at an image (that's abnormally bright, usually), the more those cells are being fired on with light. they effectively burn-in an image like a plasma tv, except the effects aren't permanent typically.
Well, sort of true. After images exist because when you look at a bright light source it causes those receptor cells to exhaust their supplies of the chemical they use to create nerve impulses, effectively becoming fatigued. When you look away from this light source you see a negative representation of this light source because the receptors that would be feeding you correct data are too fatigued to send you correct data and thus the other colors seep through from receptors that aren't fatigued in that area.
This is why those "look at this image for 30 seconds then look away and see jesus" images work. They burn a negative image of a blobular jesus into your receptors by fatiguing them, then when you look away you see the image as it's meant to be seen. This is also why if you look at one color for too long on your screen you're likely to see the opposite color when you look away.
fun fact: no google was used in the creation of this reply
