The Reversing Arrow
A Science in the Park experiment from Hayward Education Foundation. It feels like a magic trick, but it's 100% physics! Discover how a glass of water can make an arrow flip and point the other way.
Welcome to the Reversing Arrow Experiment!
Can water flip a picture backwards?
Try this quick experiment and see for yourself!
Watch the video, then follow the steps below. As the water rises in the glass, the arrow behind it seems to magically turn around. It's all thanks to refraction, the bending of light!
What You’ll Need:
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A clear, round drinking glass (smooth sides, no patterns)
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Water
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A piece of paper or a sticky note
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A bold marker (black or a dark color works best)
How to Flip the Arrow:
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Draw your arrows:
On your paper, draw two arrows that both point the same way (to the right). Put one near the top and one near the bottom. -
Set up the paper:
Stand the paper up so it is vertical, like a little sign. Lean it against a box or a stack of books. -
Position the glass:
Place the empty glass about 4 to 6 inches in front of the paper. Look through it. Both arrows still point the same way. -
Slowly pour in the water:
Keep watching through the glass as you pour. Stop once the water is above the bottom arrow, but below the top one. -
The magic moment:
Look at eye level. The bottom arrow has flipped and now points the other way! Try moving the paper closer or farther away and see what happens.
How It Works: The Science Behind the Trick!
Light travels in straight lines, but when it moves from one material into another, like from air into water, it changes speed and bends. Scientists call this bending refraction.
A round glass filled with water acts like a convex lens, the same kind of lens found in a magnifying glass. Light from the arrow bends as it enters the curved glass and bends again as it leaves. The light rays cross over each other at a point called the focal point. Once they cross, what was on the left ends up on the right, so the arrow looks like it is pointing the opposite way!
Why only the bottom arrow?
The top arrow is seen through the empty part of the glass, so its light is barely bent. The bottom arrow is seen through the water, which does all the bending. That is why only the bottom arrow flips!
Key Takeaway: Refraction Is All Around You
Lenses work by bending light, and you can find them everywhere:
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Eyeglasses and contact lenses bend light so people can see clearly.
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Cameras and phones use lenses to focus a picture.
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Microscopes and telescopes help scientists see tiny cells and faraway stars.
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Your own eyes have a lens inside that flips images too. Your brain turns them right side up!
Next time you see a straw in a glass of water, or a fish in an aquarium, remember: the light is bending on its way to your eyes!
Try This Next!
Once you've flipped the arrow, keep experimenting:
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Write a word: Write your name behind the water. Does it flip too? Try words like MOM or TOOT and see what happens!
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Change the distance: Slide the paper very close to the glass, then far away. At what distance does the arrow flip?
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Try other glasses: Does a skinny glass work differently than a wide one? What about a square container?
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Draw up and down arrows: Do arrows pointing up or down flip too? Make a prediction first, then test it!
Bonus Learning: Bending Light in Everyday Life
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Rainbows: Tiny raindrops bend sunlight and split it into all its colors.
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Swimming pools: A pool looks shallower than it really is because light bends as it leaves the water.
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Magnifying glasses: Hold one far from your eye and look at something across the room. It flips upside down, just like the arrow!
Tips for the Best Results:
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Use a smooth, round glass: Bumps, patterns, or ridges will blur the image.
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Make your arrows bold: Thick, dark lines are much easier to see through the water.
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Get down to eye level: Look straight through the side of the glass, not from above.
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Pour slowly: Watching the flip happen as the water rises is the best part!
Wrapping It Up: Magic or Science?
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It may look like a magic trick, but now you know the secret: the water in a round glass works like a lens. It bends the light, the light rays cross, and the arrow flips. Science is full of surprises like this!
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Ready for more? Head back to Science in the Park and try the Disappearing Coin and the "Broken" Straw experiments next!