Coin Illusion

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, join Cory to perform a vanishing coin illusion to explore light refraction, light travel paths, and total internal reflection. Watch the video above to see how it is done.

Disappearing coin illusion materials

For this activity you will need:

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

How to make a coin disappear: Step-by-step instructions

Step 1: Place the glass over the coin

1. Lay a coin flat on a smooth, level surface.
2. Place an empty, clear glass cup directly on top of the coin.
3. Look through the side of the empty glass and observe that the coin remains completely visible.

Step 2: Pour water into the glass

1. Keep your eyes level with the side of the glass.
2. Slowly pour water into the glass cup until it is full.
3. Watch as the coin suddenly disappears from view when viewed through the side of the glass.

Step 3: Investigate different viewing angles

1. Look straight down through the top opening of the glass. Notice that the coin is still there, sitting right under the base.
2. Look at the upper side angles to see how the surface of the water acts like a mirror, reflecting the coin upward.

Disappearing coin illusion: The science explained

This optical illusion demonstrates light travel paths, refraction, and total internal reflection:

• We see the coin because light reflects off its surface and travels through space into our eyes.
• With an empty glass, light reflecting off the coin passes through the glass base, through the air inside the glass, out through the side wall, and directly to your eyes.
• When the glass is filled with water, light travelling from the coin enters the water and hits the inner surface of the glass wall at a shallow angle. Instead of passing through the glass into the air, the light bounces off the inner surface of the glass like a mirror.
• Because the light is reflected back upward through the top opening of the glass, no light rays from the coin reach an observer looking from the side, making the coin appear to vanish completely!

Science fair projects & taking it further: Experimenting with variables

• Test whether vegetable oil, saltwater, or juice produces the same disappearance effect when poured into the glass.
• Compare a flat-bottomed glass against a curved jar or patterned cup to see how glass shape affects internal light reflection.