When there is not much light it is difficult to tell what colour things are. Challenge your students to find out more about how their eyes work.

Collect a number of different-coloured bricks or similar items. Mix them together in a box. Ask the students to find a brick that is their favourite colour. Dim the lights in the room, so you can’t see very well. Wait for about 10 minutes, using the time to talk about the activity. Ask your students to find the brick that is their favourite colour again. Turn the lights back on. How well have they done?

Our retinas are covered with light-sensitive cells called rods and cones.

When there is little light we rely on the rods to help us to see. The rods do not detect colour, therefore we find it hard to work out what colour something is in a dim room.

Get your students to form pairs and face each other. Tell them to close their eyes for 30 seconds. Then ask them to open their eyes and watch their partner’s pupils. They will see how our eyes adjust to how much light there is. In very bright conditions our irises contract to reduce the amount of light entering the eye. In dim conditions, the pupils are widened to allow more light in. When the students open their eyes the irises contract and the pupils get smaller.

Draw a red cross on a white piece of paper and stare at it for about 30 seconds. Then stare at a plain piece of paper. You will see a green cross, the same size and shape as the red one.

There are three different types of cones - each type is sensitive to a different range of colours. Staring at the red cross makes the cones detecting the red colour tired. Normally, when we look at white paper the cones are equally stimulated. However, as the cones sensitive to red have become tired they do not respond as strongly as the other types of cone and the brain sees the paper as green.

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