Hidden colours

What can you do here
Look at the still disc. Start spinning it. Slowly increase the spinning speed. What are you seeing? Experiment by spinning the disc at various angular speeds in both directions. Sometimes you will be able to see delicate pale rings on the wheel. Discuss with someone else whether both of you see the same colours. The disc we present is called Benham’s top.

How does it work
Colours are visible thanks to cone cells: photosensitive neurons located in the retina of the eye. There are three types of cones, responsive to reddish, greenish, and bluish. Other colours are seen due to several kinds of cones being activated. Black doesn’t stimulate any cone cells and white makes them all react to an extent. It is presumed that the reaction time of each cone type is different. Different cones inform the brain about black at different paces as they go from experiencing white to experiencing black. That’s why our brains interpret the image as containing other colours. We can imagine a situation when blue cones are no longer stimulated by the white part of the disc while the green and red ones still see it. The brain will construe this image as yellow.