Light travels in straight lines, but when it travels through different media it bends. This is called refraction. Here are ideas to help your pupils investigate how light travels through water, oil and air.

* Take a glass and half fill it with water. Put a pencil or ruler in the glass and look at it from different positions. Ask your pupils to record what they see. They should observe that the pencil appears bent. Light travels more slowly through water than through air. When the light enters or leaves the water it bends.

* Now add some oil to the glass, taking care to add it slowly so it forms a layer over the top of the water. Observe the pencil through the water and through the oil. Can your pupils work out whether light travels more quickly through oil or water?

* You can see refraction more clearly by making your own light-box. Take a shoe box and make a narrow vertical slit in one end. Place a torch so the light travels through the slit into the box. Dim the lights and observe where the light travels. Adjust the torch so the light travels straight through the box. Take a flat-sided glass jar and fill it almost to the top with water. Place it in the centre of the box and again observe the path of the light. Turn the jar of water at different angles and look what happens to the light.

* Once your pupils have experimented with refraction set them this challenge. Place a coin in a plastic bowl on a table. Ask one pupil to back away from the table and stop when they can no longer see the coin in the bottom of the bowl. Ask the class whether they can work out how the coin could be seen without moving the coin, the pupil or the bowl. Hopefully one of them will suggest adding water to the bowl. As you add water to the bowl the coin appears.