1. Go on. Stare at the red dot for ten seconds.
Why does it happen?
Well, when you stare at a colour for too long, the photoreceptor cells in your eye that pick up that particular colour become fatigued and fire less. This means that if you suddenly switch back to white - which triggers all the photoreceptors - the non-fatigued cells that see the opposite colour will immediately be firing more than the tired cells. And that tricks your brain into thinking there’s more of that opposite colour, which is why you see a negative afterimage.