The experiment is quick and easy and will help your students find out about the retina. The experiment can be done in pairs. Each pair will need some newspaper, felt-tip pens, and three different shapes of card in different colours (for example, a red triangle).

Cover the floor with a couple of pieces of newspaper or lining paper. The first student should stand on the edge of the paper, half way along. The second should draw a semicircle of radius 1 metre on the newspaper in front of their partner.

While the first student looks straight ahead, the second holds up one of the shapes and walks around the semicircle, starting from one end. The first student says when they notice any movement and when they can determine the shape and its colour. These things will happen at different points around the semicircle, and the second student should mark where they happen.

The students then swap places and repeat the experiment, making sure that they mark the result on the semicircle with a different coloured pen.

The retina, which is at the back of the eye, is covered with light-sensitive cells. There are two basic types of cell - rods and cones.

It is the cones that enable us to see colour, and these are concentrated at the centre of the retina. When we see things in the periphery of our vision the light is being focused onto the edges of our retina, where there are mainly rods. These cells enable us to detect motion. When the shape is nearer the centre of the semicircle the image is focused on the rods and cones in the centre of the eye, enabling us to determine the colour of the shape.

* Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn