Gizmodo posted a rather trippy optical illusion yesterday where the green and the blue in this spiral are actually the same color. As interesting as that is (the explanation is here), one of the commenters submitted another illusion that caught my eye.
The Benham top, created by nineteenth-century British toymaker Charles Benham, is a disc that contains a black-and-white pattern, which when spun gives the illusion of colour. These colours are visible on different parts of the disk, and not everyone see the same colour.
I see red quite clearly. What about you?
The answer to why different people see different colours on Benham’s top is not a concrete one. Hit the jump to read one of the theories.
Different people see different intensities of colors on this spinning disk. Just why people see color here is not fully understood, but the illusion involves color vision cells in your eyes called cones.
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 color, 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 (have the longest latency time), and keep responding the longest (have the longest persistence time).
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. But your eyes and brain see the color 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 partly explains why you see colors.
The colors vary across the disk because at different radial positions on the disk the black arcs have different lengths, so that the flashing rate they produce on the retina is also different.
The explanation of the colors produced by Benham’s disk is more complicated than the simple explanation outlined above. For example, the short black arcs that are on all Benham’s disks must also be thin, or no colors will appear.