Coin Illusion

Materials

For this activity, you will need:

• A coin
• A clear, smooth glass with a flat bottom
• Water
• A jug or pitcher filled with water

How to make a coin disappear

Step 1: Place the glass over the coin

1. Place a coin on a smooth, level surface.
2. Position an empty glass directly over the coin.
3. Look through the side of the glass and observe the coin.

Step 2: Fill the glass with water

1. Keep your eyes level with the side of the glass.
2. Slowly pour water into the glass until it is nearly full.
3. Watch carefully as the coin appears to disappear from view.

Step 3: Investigate different viewing angles

1. Look directly down through the top of the glass.
2. Observe that the coin is still resting beneath the glass.
3. Move your head to view the glass from different angles.
4. Notice how the appearance of the coin changes depending on where you look.

How does it work?

This illusion demonstrates how light changes direction as it travels through different materials.

We see objects when light reflects from them and enters our eyes.

With an empty glass:

• Light reflected from the coin passes through the glass and air.
• The light travels directly to your eyes.
• The coin remains visible.

When the glass is filled with water:

• Light from the coin travels through water before reaching the glass wall.
• Some of the light changes direction as it moves between water, glass and air.
• Some of the light is reflected inside the glass instead of travelling directly to your eyes.
• Fewer light rays from the coin reach an observer viewing from the side.

As a result, the coin can appear to vanish even though it is still sitting beneath the glass.

This effect is caused by the way light interacts with different materials and demonstrates important principles of optics, including refraction and reflection.

Science fair project: Take it further

Once you have made the coin disappear, investigate how different variables affect the illusion.

• Compare water with other liquids such as salt water, vegetable oil or juice. Do they produce the same effect?
• Test different glass shapes, such as a straight-sided glass, jar or curved cup. Which shape creates the strongest illusion?
• Try using coins of different sizes. Does the size of the object affect how easily it disappears?
• Compare different viewing angles. At which angle does the coin disappear most effectively?
• Create a table to record your observations. Which liquid or container produced the strongest disappearing effect? What patterns can you identify?