The Disappearing Penny: A Tabletop Lesson in Light Refraction
What Makes a Penny Disappear in Plain Sight?
The Disappearing Penny experiment demonstrates how light bends: or refracts: when it passes through water, creating an optical illusion that makes a coin vanish right before your eyes. By placing a penny in a shallow bowl or cup and adding water, you'll watch as physics performs a magic trick: the coin seems to disappear from certain viewing angles, then reappears when you move your head. This simple demonstration reveals the fundamental behavior of light waves as they travel through different materials, and it takes less than a minute to set up.
This experiment sits at the sweet spot of science education: quick enough to hold a young learner's attention, dramatic enough to spark genuine curiosity, and scientifically rich enough to launch deeper conversations about how light behaves in our everyday world. Whether you're homeschooling, looking for a rainy afternoon activity, or prepping for a science fair, the Disappearing Penny delivers that satisfying "aha!" moment that makes kids (and adults) want to understand why it works.
The Wow Factor of Bending Light
Here's what makes this experiment particularly engaging: everyone assumes they understand how seeing works. Light travels in straight lines, hits objects, bounces back to our eyes: simple, right? Then you pour water into that bowl, and suddenly the rules seem to break. The penny vanishes. Move your head slightly, and it reappears. The dissonance between expectation and observation creates that perfect teaching moment.
Unlike experiments that require expensive materials or complex setups, this one leverages items you likely have within arm's reach right now. The barrier to entry is practically zero, yet the conceptual payload is substantial. You're not just making something disappear: you're demonstrating the same principle that makes swimming pools look shallower than they are, creates mirages on hot highways, and enables fiber optic cables to transmit information at light speed.
Materials & Cost Breakdown
One of the beautiful aspects of the Disappearing Penny is its accessibility. Here's what you'll need:
| Item | Typical Cost | Notes | Where to Find |
|---|---|---|---|
| Shallow bowl or drinking glass | $0–$3 | Clear glass or plastic works best; must see through sides | Kitchen cupboard or dollar store |
| Penny (or any coin) | $0.01 | Shinier coins show effect better | Pocket change |
| Water | $0 | Tap water works perfectly | Kitchen sink |
| Optional: Food coloring | $1–$2 | Makes the water more visible in photos | Grocery store baking aisle |
Total estimated cost: $0.01–$6 (if you need to purchase everything)
Most families will spend exactly one cent on this experiment, assuming you already own a drinking glass. That's tough to beat for educational value per dollar.
Step-by-Step Instructions
Setup Phase
1. Choose your viewing vessel. A short, wide bowl or glass works better than a tall, narrow one. You want something where you can easily see the bottom when looking through the side. A small cereal bowl or a rocks glass are both excellent choices.
2. Position the penny. Place your coin in the center of the empty bowl. Step back slightly and crouch down until your eyes are just below the rim of the bowl: you should see the penny clearly through the side of the glass.
3. Mark your viewing angle. This is optional but helpful, especially with younger kids. Note exactly where you're standing and where you're looking. Some families like to put a piece of tape on the floor to mark the "magic viewing spot."
The Disappearing Act
4. Maintain your position. Keep your head in exactly the same spot where you can see the penny through the side of the bowl.
5. Slowly add water. Have a helper pour water into the bowl while you watch, or carefully pour it yourself while maintaining your viewing angle. Pour gently to avoid splashing the coin out of position.
6. Watch the transformation. As the water level rises, the penny will gradually fade from view. At a certain depth: usually when the water covers the coin by an inch or more: it will completely disappear from your viewing angle.
7. Test the illusion. Stand up and look straight down into the bowl. The penny is still there, perfectly visible from above. Return to your side-viewing position, and it vanishes again.
Taking It Further
8. Try different angles. Move your head up and down slightly. You'll find a specific range of angles where the penny is invisible, and others where it suddenly pops back into view.
9. Experiment with the water level. Drain some water out slowly. At what level does the penny reappear? This helps kids understand that the effect is tied directly to the presence of water and the viewing angle.
10. Test different containers. Does the shape of the bowl matter? Try a tall glass versus a wide, shallow dish. The penny will disappear in both, but the angles required might differ.
The Science Behind the Vanishing Act
So what's actually happening here? The answer lies in a phenomenon called refraction: the bending of light as it passes from one transparent material into another.
Light Travels at Different Speeds
Light moves at different speeds through different materials. In a vacuum, it zips along at about 300,000 kilometers per second. In air, it's just slightly slower. But in water, light slows down to about 75% of its speed in air. This slowdown causes the light waves to bend.
Think of it like a car driving from pavement onto sand at an angle. The wheel that hits the sand first slows down while the other wheel is still on the pavement, causing the car to turn. Light does something similar when it hits the water's surface at an angle.
The Critical Angle and Total Internal Reflection
When light travels from the penny upward through the water, it bends as it hits the water-air boundary. At shallow viewing angles: when you're looking through the side of the bowl: the light from the penny would need to bend extremely sharply to reach your eyes.
But light can only bend so far. Beyond a certain angle (called the critical angle), the light doesn't exit the water at all. Instead, it reflects back into the water, bouncing off the surface like a mirror. This is total internal reflection, and it's the reason the penny completely disappears rather than just looking distorted.
The light rays that would normally carry the image of the penny to your eyes get redirected by this reflection, never making the journey from the coin to your retina. Result? The penny vanishes.
Why You Can Still See It from Above
When you look straight down into the bowl, you're viewing the penny at a perpendicular angle to the water's surface. At this angle, light from the penny doesn't hit the critical angle for total internal reflection. The light bends slightly as it exits the water, but it still reaches your eyes, so you see the coin clearly.
This same principle explains why fish in an aquarium can see out in all directions, but from certain angles outside the tank, you can't see deep into the water: the light reflects instead of transmitting through.
Variations and Extensions
The Reappearing Coin
Once the penny has disappeared, try this: slowly tilt the bowl toward you while maintaining your viewing position. As the angle of the water's surface changes relative to your eyes, the penny will suddenly reappear. This variation really drives home that the effect is all about angles.
Multiple Coins
Place several pennies in the bowl at different distances from you. Add water and observe: do they all disappear at once, or in sequence? This helps kids understand that distance from the viewer affects the viewing angle and, therefore, when each coin vanishes.
Different Liquids
Try the experiment with cooking oil instead of water. Oil has a different refractive index than water, so the bending effect differs slightly. Does the penny disappear at the same water level with oil? This introduces the concept that different transparent materials bend light by different amounts.
The Laser Pointer Extension
For older kids or middle schoolers, add a small laser pointer to the experiment (with appropriate safety supervision). Shine it through the side of the empty bowl, then add water while keeping the laser at the same angle. You'll actually see the beam of light bend as it enters the water, making the invisible force of refraction visible.
Real-World Applications of Refraction
Understanding refraction isn't just about making pennies disappear: it's about understanding how light behaves in countless real-world situations:
Swimming pools appear shallower than they actually are, which is why we teach kids to check depth markers rather than trusting their eyes.
Eyeglasses and contact lenses work by using precisely shaped pieces of glass or plastic to bend light in ways that correct vision problems.
Cameras and microscopes depend on carefully designed lenses that use refraction to focus light and create clear images.
Fiber optic cables use total internal reflection (the same principle that makes the penny disappear) to trap light inside thin glass fibers, allowing it to travel long distances while carrying internet data, phone calls, and television signals.
Mirages on hot highways occur because light bends when it passes through layers of air at different temperatures, creating the illusion of water on the road ahead.
When kids understand that the same principle making their penny vanish is also carrying their favorite YouTube video through fiber optic cables at near-light speed, science suddenly feels a lot more relevant to their daily lives.
Common Questions About the Disappearing Penny
How does the size of the container affect the experiment?
Larger, wider containers give you more room to experiment with viewing angles and water depths, but the basic principle works in any clear vessel. Taller containers require more water to achieve the disappearing effect, which can help demonstrate that the amount of water (and therefore the distance light travels through it) matters.
Why doesn't the penny disappear when I look straight down?
The disappearing effect depends on viewing angle. When you look straight down, light from the penny travels nearly perpendicular to the water's surface, which doesn't create the critical angle needed for total internal reflection. The light bends slightly but still reaches your eyes.
Can I use other objects besides a penny?
You can use any small, flat object: quarters, buttons, small toys, or even a drawing on paper. Shiny, high-contrast objects tend to show the effect most dramatically because they're easier to see when present and their absence is more striking.
Does the temperature of the water matter?
Not significantly for this basic experiment. Temperature affects water's refractive index slightly, but you'd need precision equipment to measure the difference. Room temperature tap water works perfectly well.
Why does the penny look slightly distorted before it completely disappears?
As you add water, you're seeing partial refraction effects before you reach the critical angle for total internal reflection. The light from the penny is bending, making it appear in a slightly different position than it actually occupies, similar to how a stick appears bent when you put it partway into water.
Can kids do this experiment independently?
For children ages 6 and up, this is generally safe to do independently with minimal supervision. The only "risk" is minor water spillage. Younger children (ages 4-5) may need help pouring the water steadily, but they can certainly observe and participate in the experiment.
Connecting to Other Refraction Experiments
The Disappearing Penny works beautifully as part of a refraction series. Kids who enjoy this demonstration often want to try:
- The Bent Pencil experiment, where a pencil in a glass of water appears broken or bent at the water line
- The Reverse Arrow Illusion, where an arrow drawn on paper appears to flip direction when viewed through a water-filled glass
- Making rainbows with a glass of water and sunlight, showing how refraction can split white light into its component colors
Each of these builds on the same fundamental concept: light bending as it passes through different materials: while providing different visual results. Together, they create a comprehensive picture of how refraction shapes what we see in the world around us.
Tips for Teaching This Experiment
Encourage prediction. Before adding water, ask kids what they think will happen. Most will guess that the water might magnify the penny or make it look wavy: few predict it will vanish entirely. This surprise creates a powerful learning moment.
Let them control the variables. Hand the pitcher to the child and let them pour. Let them choose the viewing angle. The more they direct the experiment, the more engaged they'll be with understanding the results.
Ask "why" questions without immediately answering them. "Why do you think the penny disappeared?" "What changed between when you could see it and when you couldn't?" Give kids time to develop their own theories before offering the scientific explanation.
Use comparison language. Talk about light "traveling through" air versus water, light moving "fast" versus "slow." These concrete comparisons help younger learners grasp abstract concepts like refraction and speed of light.
Connect to their experiences. Ask if they've ever noticed that pools look shallower than they are, or if they've seen something that looked bent underwater. Building bridges between the experiment and real-world observations helps cement understanding.
Why This Experiment Matters
In an age of digital entertainment and passive screen time, hands-on science experiments offer something increasingly rare: the chance to be surprised by the physical world. The Disappearing Penny doesn't require batteries, doesn't have a screen, and can't be swiped or clicked. It requires observation, manipulation of physical objects, and genuine thinking about cause and effect.
More importantly, it teaches a fundamental truth about science: things aren't always what they seem. Our eyes can be fooled, our intuitions can be wrong, and understanding why something happens often reveals that the world is even more interesting than it appears on the surface.
That lesson: delivered through the simple act of watching a penny vanish in a bowl of water: might seem small. But it's the foundation of scientific thinking: observe carefully, question your assumptions, and seek to understand the mechanisms behind what you see.
For a one-cent investment and sixty seconds of time, that's a lesson worth teaching.
Disclaimer: This experiment involves common household water and should be conducted with appropriate supervision based on the child's age and developmental level. Adult oversight is recommended when young children are handling water to prevent spills and ensure safe participation. The information provided is for educational purposes and should not replace proper adult guidance during science activities. Tierney Family Farms is not responsible for any outcomes from attempting experiments described in this blog post. Always consider your child's individual abilities and adjust supervision accordingly.