Invented in the 19th century by a toy maker, Benham's disk provides an opportunity to explore how our eyes work.
You can buy a Benham's disk from a toy shop, or you can make your own.
To make your own, create a spinner from a piece of card and a cocktail stick. Cut a circle of card about 6cm in diameter. Make a small hole in the centre. Break the cocktail stick in half and cover the broken end with Sellotape. Push it through the hole in the card, and see if the disk spins well.
Remove the cocktail stick and use the card as a template to cut out a number of white paper disks. Draw a line down the centre of each white paper disk, and colour half the disk black. Make a pattern on the white half of the disk. The pattern should consist of concentric circles that are not continuous but broken up. (There are lots of internet sites where you can download patterns for your Benham's disk or you can copy the one shown here.) You may need to experiment to get a pattern that works. Fix the pattern to the top of the card and recreate the spinner. What do you see when you spin the disk in a brightly lit area?
You should find that you see colours despite the fact the pattern is black and white. Different people see the colours differently and the illustration is just one example. The reason this happens is not fully understood. However, it is thought to be a result of how the cones in our eyes work. Cones are sensitive to different colours - red, green and blue.
However the cones take different lengths of time to respond to light. White light is made up of all three colours, however you see white light only when the cones are all responding together. The flashing white and black light caused by the rotating disk and the broken circles, means that the difference in response time of the cones becomes noticeable as colours.
A website at www.exploratorium.edusnacksbenhams_disk.html is useful for teachers but note that it's for adults; it describes using power tools to spin the disk.