Benham's disk is a simple illusion with a spinning circle, half black, half white streaked with short black lines. There is no color involved, and yet, when the disk spins, people can see bright streaks of it swirling in the top. Why?
One of the easiest things to discern is different colors. People who can't even focus can tell a black and white world from a colorful one. And yet there's an optical illusion that plays tricks with even that most basic sense. Benham's disk, also called Benham's top, is a disk with a pin through the center. Half the disk is solid black. Have is white with black line segments that describe arcs of concentric circles. Spin the disk on the pin, and many people will see a sort of striped whirling circle. It will continue to be black, white, and shades of gray, until it reaches certain speeds. Those speeds vary from person to person - although about three to five revolutions per second is considered best - but suddenly those lines will change and people should see streaks of green, blue, red, and yellow.
The toy was invented in 1894 by C. E. Benham, but it's hard to say exactly what makes it work even now. There are theories, though. The most plausible one involves the visual receptors in the eye. Rods see black and white, but are best for seeing in low light. Cones need a little more light, but they see color. Darkness - the black areas of the disk - deactivates cones. White light activates all the cones in the eye. It's possible, thought, that different cones take different amounts of time to deactivate. So a cone that sees a certain color will switch off fast when it is flashed with black, but others will keep going for a little while. Since one of the cones drops out, instead of seeing neutral white - the combination of colors - we see the color that the leftover activated cones receive.
Top Image: Jorge Barrios