This "after-image" happens because the images we "see" are created by the brain based on signals that our eyes send. Usually what we see is pretty accurate, but our brain can be fooled too.

If you could open an eyeball, you would see that it is a hollow ball (or sphere) filled with liquid. Check out the picture here: anatomy of the eye .

Light comes in the pupil at the front and hits the retina at the back. The retina contains lots of tiny sensors that pick up light. Rods are the sensors that allow us to see black and white and work very well when there's not much light. We're interested in the cones today because they allow us to see color. (Why do you think cats are colorblind? Think about when cats do their hunting.)

When a particular color of light hits a particular spot in the retina, a particular cone sends a message to the brain. Let's say it's a cone in the center of the retina that responds only to red light. Take a look at the red dot in the green field at this site: here . That one cone sends a message to the brain saying "there's something red in the center of your vision". So do all of the many red cones around it. The chemical reactions responsible happen incredibly fast. Then it takes the cone a short time to recover. You will still see red, but some cones are recovering while other cones are sending the signal. It's like taking turns with your friends to get a big job done. You keep working, but all of you get tired. Now when you look away, the red cones are all sending weaker signals.

White light is made up of all the colors. (You can test this by using a prism to separate white light into a "rainbow.") When you look at something white, all of the cones in your eye send a signal, but now your red cones send a weaker signal. The blue and green cones send a strong signal, so the white area looks like it has a blue green dot in the center.

Think about it this way, let's say that everyone in your class were assigned a color. When your teacher held up a paper with your color on it, you would yell out that color. If your teacher held up a red paper for a long time, you would get tired of yelling. Then when the teacher held up a paper containing all of the colors, all of the blue and green kids would be able to yell their colors loudly, while you would barely be able to say "red." A person listening to your class would think the paper was blue or green.

You might be getting confused, saying, "When I mix all of my paint colors together, I get brownish black, not white." I hope you did, because that shows that you are thinking for yourself and comparing what I say to what you have seen. The reason you get brown/black when you mix paints is because you are mixing "pigments." What your eye sees is the light that is reflected off the pigments. All pigments mixed together would give you black. All of the light is absorbed by a black object and none is reflected back to your eye. (That's why black objects heat up so fast, they absorb light.) A white object reflects all of the colors back to your eye instead of absorbing them. Your class might want to experiment with this idea. All you need are lamps with colored light bulbs or flashlights with colored plastic over them.

This question has everything to do with how your eye sees color. I'm not an expert in biology but I will try to answer your question.

In the back of your eye, there are cells specifically designed to detect light of one particular color. Now, there are infinitely many colors so it would seem that you would need a lot of cells to be able to see all the colors that you see. Your eye is very tricky, though. You have cells which are designed to pick up only red, green, and blue light. If you are seeing a color in between red and green, like yellow, your eye will pick up a little red and a little green. Your brain can then take that information and combine it to make yellow.

This explains how color TVs work, by the way. There are lots of tiny red, green, and blue dots on a TV. By turning on both red and green, the red and green cells in your eye are triggered and your brain thinks that you are seeing yellow light.

If you spend a long time looking at something red, the cells in your eye that are supposed to see red get "tired". Of course, they don't really get tired the way your muscles get tired. I think what happens is that your brain is designed to recognize colors no matter what color of light you shine on them. A green leaf looks green whether you put it under a reddish light or a yellowish light. In fact, the light from the sun is not white light, it is yellow light. Your eyes are adjusted to that, though, so white objects still look white.

When you look at something red for a long time, the cells in your eye adjust by becoming less sensitive to red light. Now, when you suddenly look away from the red, your green and blue cells are more sensitive than your red cells and you end up seeing a greenish-blue spot.

Good questions! I've asked that this question get sent to a biologist. They would know more about how the brain and eyes work. Maybe they can give you a better reason why your cells become less sensitive when they see the same color too long.