Vanishing coin

In this activity students see how total internal reflection makes a coin seem to vanish.

Learning outcome

Students can explain why a coin under a beaker of water is not visible when viewed through the side of the beaker.

Equipment

Each student will need:

• A small empty beaker (or drinks 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 upside down on the paper. Draw around it and cut out to make a lid.
2. Cut a hole in the lid so that water can be poured through it.
3. Place the coin on bench, making sure that both are completely dry.
4. Place the beaker on top of the coin and add the lid.
5. Pour water into the beaker so that it is completely full. The coin should no longer be visible.
6. Lift the lid. An image of the coin should be now be visible on the inside vertical surface of the beaker.

Discussion prompts

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

Explanation

For the empty beaker, the light refracts but still passes through the side of the glass. For the full glass, light cannot escape because of total internal reflection and so the coin seems to vanish to anyone viewing it through the side of the beaker.

Teaching notes

This trick works because a full beaker forms a five layer air-glass-water-glass-air structure. The first layer of air is created by a gap between coin and bottom of beaker due to the ridges on the coin. If the coin is wet, it will not vanish. Students can check this for themselves by putting a few drops of water on the bottom of their beaker.

Material	Refractive index
Air	1.00
Water	1.33
Glass	1.51

This experiment was safety-checked in March 2020.