Hidden colours

What you can do here
Look at the still disk and start spinning it gradually increasing the speed. What can you see? Probably delicate pale patterns on the disk. Ask someone else whether both of you see the same colours. The disk is called Benham’s top.

How does it work
You can see colours thanks to cone cells, i.e. photosensitive neurons located in the retina of the eye. There are three types of cones, responsive to reddish, greenish, and bluish colour. You can see other colours thanks to the activation of several kinds of cones. Black doesn’t stimulate any cone cells and white makes them all react to an extent. It is thought that the reaction time of each cone type is different. Different cones inform the brain about the black at various paces as they go from sensing white to black. That’s why your brain interprets the image as containing other colours. Imagine a situation when blue cones are no longer stimulated by the white part of the disk, while the green and red ones still see it. The brain will see this image as yellow.