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 .
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
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.