The Disappearing Coin

A Science in the Park experiment from Hayward Education Foundation. Now you see it, now you don't! Learn how a glass of water can make a coin vanish right before your eyes.

Welcome to the Disappearing Coin Experiment!

Can water make a coin disappear?

Try this quick experiment and see for yourself!

Watch the video, then follow the steps below. Put a coin under a glass, add water, and look through the side. The coin vanishes! It's all thanks to the way light bends and bounces inside the water.

What You’ll Need:

A clear drinking glass
A coin (any kind will work)
Water
A small plate or saucer (optional)

How to Make the Coin Disappear:

Place the coin:
Put the coin flat on a table.
Cover it with the glass:
Set the empty glass right on top of the coin. Look through the side of the glass. You can still see the coin, right?
Slowly pour in the water:
Fill the glass almost to the top. Keep your eyes on the side of the glass as you pour.
The magic moment:
Look through the side again. The coin has disappeared! For an even bigger surprise, set a saucer on top of the glass so no one can peek in from above.

How It Works: The Science Behind the Trick!

We see things when light bounces off them and travels to our eyes. Before you add water, light from the coin travels up through the air, passes through the side of the glass, and reaches your eyes. You can see the coin!

When you add water, the light from the coin has to travel from the water into the glass wall and out into the air. Each time light passes from one material to another, it bends. This is called refraction. The light from the coin hits the side of the glass at such a low, slanted angle that it can't escape. Instead, it bounces back into the water like a mirror!

The big science word:
When light bounces back instead of passing through, scientists call it total internal reflection. Since no light from the coin reaches your eyes through the side of the glass, the coin seems to vanish. It's still there. The light just can't get out to show you!

Key Takeaway: Trapping Light Is Super Useful

Scientists and engineers use this same trick to trap light on purpose:

Fiber optic cables are thin strands of glass that carry internet, TV, and phone signals as flashes of light. The light bounces along the inside of the cable, just like in your glass, without ever escaping.
Doctors use tiny light cameras called endoscopes to see inside the body.
Diamonds sparkle because light gets trapped and bounces around inside them.

So the next time you stream a video, thank total internal reflection!

Try This Next!

Once you've made the coin vanish, keep experimenting:

Look from above: Peek straight down into the water. Can you see the coin from the top? Why do you think that is?
Try different glasses: Does the trick work with a thick glass, a thin glass, or a plastic cup?
Wet the coin: Put a few drops of water under the glass so it sits in a puddle. Does the coin still disappear?
Hide a message: Write a tiny secret word on paper and put it under the glass instead of a coin!

Bonus Learning: Bending Light in Everyday Life

Rainbows: Light bends and bounces inside raindrops, splitting into all its colors.
Swimming pools: Look up from underwater and the surface can look like a shiny mirror. That's total internal reflection too!
Aquariums: Look at a fish tank from the corner and you may see the fish reflected, or not see them at all!

Tips for the Best Results:

Use a smooth, clear glass: A plain glass with straight sides works best.
Keep the bottom dry: The coin should sit on a dry table, with a thin layer of air under the glass.
Look from the side: Get down low and look through the side of the glass, not down from the top.
Fill it up: The trick works best when the glass is nearly full.

Wrapping It Up: Magic or Science?

It may look like a magic trick, but now you know the secret: the light from the coin gets trapped and bounces back inside the water, so it never reaches your eyes. The coin was there the whole time!
Ready for more? Head back to Science in the Park and try the Reversing Arrow and the "Broken" Straw experiments next!