The year 10 class at Marlborough Girls' College asks :-

A colour wheel consists of a series of black lines which, when spun, produces colours. Why is this so?

Gordon Sanderson, an opthalmologist at the University of Otago's School of Medicine, responded.

The question about the colour wheel is an interesting one. I think most people are familiar with Maxwell’s disc; it is something most of us made at school. It consists of a disc of cardboard with two holes in the middle through which a loop of string is passed. By winding up the string and pulling with both hands the disc can be made to spin at very high speeds. If the disc is ruled into sectors and each sector painted with the primary colours: red, green and blue in a reasonably even distribution, once the disc is spun it can appear to turn white. This was one of the ways that a very famous scientist called James Clerk Maxwell was able to demonstrate Newton’s theory that white light is a mixture of colours.

However the colour wheel that you describe requires a different explanation; if the disc is painted white and some black radial crescents of different length are painted within it, once it is spun at high speed it is possible to perceive the crescents changing colour. This is called Benham’s top and it was first made by a toymaker in the 19th century.

Nobody really understands how it works, but it seems to rely on the fact that some of the processing of colour vision takes place in the retina before the information is transmitted via the optic nerve to the visual cortex of the brain. There may be a small time delay between the different nerve cells that are responsible for transmitting the stimulus from the three types of cones that we normally use to see colour. This would mean that the brain is “fooled” into seeing colours that do not actually exist. There is almost certainly a neurological component to this effect because different people see different colours.