Benham's Disk

In 1894, toymaker Mr. C.E. Benham discovered that a spinning disk with a particular pattern of black and white marks could cause people to see colors. Mr. Benham called his disk an "Artificial Spectrum Top" and sold it through Messrs. Newton and Co. Benham's Top (or Benham's Disk) has puzzled scientists for over 100 years.

What's Happening? What Causes the Colors?

The retina of the eye is composed of two types of receptors sensitive to light: cones and rods. Cones are important for color vision and for seeing in bright light. There are three types of cones, each of which is most sensitive to a particular wavelength of light. Rods are important for seeing in low light.

It is possible that the colors seen in spinning Benham disks are the result of changes that occur in the retina and other parts of the visual system. For example, the spinning disks may activate neighboring areas of the retina differently. In other words, the black and white areas of the disk stimulate different parts of the retina. This alternating response may cause some type of interaction within the nervous system that generates colors.

Another theory is that different types of cones take different times to respond and that they stay activated for different amounts of time. Therefore, when you spin the disk, the white color activates all three types of cones, but then the black deactivates them. The activation/deactivation sequence causes an imbalance because the different types of cones take different times to respond and stay on for different times. This imbalance in information going to the brain results in colors.

Neither of these theories explains the colors of Benham's disk completely and the reason behind the illusion remains unsolved. 

ANother explanation

Benham's Disk was invented by a nineteenth-century toymaker who noticed colours in a black-and-white pattern he had mounted on a spinning top.

Why do we see colours? There are three types of cones. One is most sensitive to red light, one to green light, and one to blue light. Each type of cone has a different latency time, the time it takes to respond to a stimulus, and a different persistence of response time, the time it keeps responding after the stimulus has been removed. Blue cones, for example, are the slowest to respond and keep responding the longest.

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. When a white flash goes by, all three types of cones respond. Your eyes and brain see the colour white only when all three types of cones are responding equally. The fact that some types of cones respond more quickly than others and that some types of cones keep responding longer than others leads to an imbalance that partially explains why you see colours.

The colours vary across the disk because the black arcs have different lengths, so that the flashing rate they produce on the retina is also different.