Question: Why is it that when you look at something for a long time , when you look at a blank surface the object is still there but in a different colour and blurry? Why do we sometimes get blurry spots on our eyes?

Rebecca Dewey answered on 17 Jun 2015:

You have two different kinds of cells that detect light reaching the retina, in the back of the eye: rods and cones. Rods detect light intensity and cones detect colour. You have more rods around the edges of the visual field and more cones in the middle. When you look at at something, these cells detect the light by using a pigment (dye) and send the message down the optic nerve to the brain. If you keep looking at it, the cells get used to the object being there and don’t send the signal so strongly anymore as they run out of pigment. Then when you look away, the cells suddenly register a change in the light that reaches them and send the new signal, but this is likely to over compensate as the cells have already adapted to the light levels. This will pass once the cells have replenished their supply of pigment.