Colours that aren't There!

The display consists of three discs with a variety of black and white patterns. When the discs are rotated, colors appear.

The apparent explanation is that the eye's color receptors respond on different time scales to the flashes of white light produced by the rotating disc.

At each point in space, we see alternating black and white patches. When a white patch flashes before our eyes, our color receptors respond at different rates and intensities to the various colors that appear. The effect was discovered in 1895 by an English toymaker, Charles Benham, while he was making tops with black and white patterns.

The eye has three types of light receptors for color: one for green light, one for blue light, and a third that is highly sensitive to red light. Therefore, when we see a red object, the red receptors are the most active, and the eye and brain identify the object's color as red. White, on the other hand, a combination of three colors, sets all three receptors into operation to a similar degree.

According to one theory, the white in the Benham's Top Illusion is seen to have flashes of color because of another difference between the three types of receptors: their reaction times when a light goes on and when it goes off. Researcher have noted, for example, that when a white light is flashed, the receptor for blue light is the last to start reacting and the last stop reacting. Staring at the rotating disc is like staring at a series of white flashes. The situation leads to an imbalance in the operation of the receptors and creates an illusion of color.

Link:
From a website for children on brain research: Another way to make a top, and a special survey to take.
http://faculty.washington.edu/chudler/benham.html

Related exhibit:
Velvet Wire Net