Vanishing coin

In this activity students see how a coin under a beaker of water can’t be seen through its side. You can use it to introduce total internal reflection.

Equipment

Each student will need:

• A small empty beaker (or glass) with straight sides
• A larger jug/beaker of water
• A coin
• Paper or card
• Scissors
• Tissue or cloth to mop up spillages

Procedure

Ask students to:

1. Put the beaker/glass upside down on the paper. Draw around it and cut out to make a lid.
2. Place the coin on bench/table, making sure that both are completely dry.
3. Place the glass on top of the coin.
4. Pour water into the glass so that it is completely full.
5. Add the lid. The coin should not be visible when viewed through the side of the glass.
6. Lift the lid. They should be able to see an image of the coin (or multiple images if they are using a tall glass).

Discussion prompts

• What needs to happen for us to be able to see the coin?
• Where does the light go when the glass is full?

Teaching notes

It is important to ensure that the coin is completely dry in this experiment. Ridges on the coin create a small air-gap between bottom of glass and coin. Refraction as light travels from the air to the glass ensures that the angle of incidence at the side of the glass is larger than the critical angle. If the coin is wet, the coin will not vanish.

Discuss how light from the coin needs to enter our eye for us to see it. Provide ray diagrams for the empty glass, showing the light passing through the side of the glass, and the full glass, where light cannot escape because it reflects. Introduce the term ‘total internal reflection’.

Learning outcome

Students understand that total internal reflection can occur when light is travelling from water (or glass) to air.

This experiment was safety-checked in March 2020.