This is an experiment that takes a lot of patience, but is well worth it. Look at the flag below. It looks familiar, but something is a bit off...
Now, we want you to stare at the flag for thirty seconds, without blinking or moving your eyes. Pick a spot near the center of the flag that you can focus on, and stare. Hard. For thirty seconds. Don't blink. Maybe you should sing a verse of the Star Spangled Banner just to amuse yourself...
After you are done staring, look at a white wall, and stare at it. You may want to blink a few times. Do you see the flag a bit better now? You should see an afterimage that is in all the right colors!
So what is happening? When you stare
at a colored image for a long time, the color sensors in your
eyes get tired. And so, when you look at a white surface, they
tend to see the color that is the opposite of the color they were
just looking at. The word for the opposite color is the complementary
color. The flag is red, white and blue; the complementary colors
to these are cyan (a mix of blue and green), black and yellow
(actually a mix of red and green). These are just the colors that
we used to make the flag above. And so there you have it - in
the afterimage of this odd flag, the flag shows up in its usual
mix of red, white and blue!