- Age:5+
- Time:10 min
- Difficulty:Easy
- Mess level:Low
- Supervision:Yes
🧒 In one sentence
A round glass of water acts like a lens, bending light so a sideways arrow held far enough behind it appears to point the other way.
Materials Needed for the Refraction Demonstration
A bold, asymmetrical drawing makes the left-to-right change easy to spot.
- A clear, round-sided drinking glass
- Water
- White paper or an index card
- A dark marker
- Optional: a ruler
What will happen when you view a sideways arrow through the water-filled glass?
Make your prediction, then tap an answer to check!
Instructions for the Refraction Demonstration
👨👧 Adult supervision needed
An adult should supervise young children around glass. Set the glass on a stable surface, carry it with two hands and wipe up spills immediately.
- Draw a thick sideways arrow on the paper. Make one end clearly different from the other.
- Fill the round glass almost to the top with water and place it on a level surface.
- Hold the arrow upright behind the glass, at the same height as the water. Look through the curved side of the glass from the opposite side.
- Begin with the card close to the glass, then slowly move it farther away. Keep your head and the glass still.
- Stop when the arrow appears to point in the opposite direction. Measure that glass-to-card distance if you want to compare trials.
- Move the card closer and farther several times. Notice where the image is magnified, blurred or reversed.
If the arrow does not flip
Use a round, smooth-sided glass and view through the water-filled part, not above the waterline. Draw a larger arrow, close one eye, add a plain background and move the card through a wider range of distances. Square glasses and patterned or faceted sides distort light differently and may not produce a clean reversal.
Why Does the Arrow Flip?
Light from the drawing travels through air, curved glass, water, glass again and finally air before reaching your eyes. At each angled boundary, its direction can change. Together, the round glass and water behave like a cylindrical converging lens: they focus light from side to side much more strongly than from top to bottom.
When the arrow is far enough behind this lens, rays coming from its left and right sides cross before reaching you. Your eye receives the left-side rays from the right and the right-side rays from the left, so the arrow appears horizontally reversed. This is why a sideways arrow or a short word works better than a circle, which looks unchanged after a left-to-right flip.
The exact flip distance is not universal. It depends on the glass's curvature and thickness, the water, and the distance between the drawing, glass and viewer.
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See what you’ll get →What Is Refraction?
Refraction is a change in a light ray's direction when it crosses into a material where its speed is different. The refractive index is written as n = c/v, where c is the speed of light in a vacuum and v is its speed in the material. Air has a refractive index of about 1.0003 and water about 1.333, so light travels through water at roughly three-quarters of its vacuum speed.
At a flat boundary, a ray entering water from air at an angle bends toward the normal, an imaginary line perpendicular to the surface. It bends away from the normal when it leaves water for air. A ray that hits the surface straight on changes speed but not direction.
This is about optical properties, not simply how much matter is packed into a volume. Physical mass density and refractive index are different measurements; a material that is physically denser does not automatically bend light more.
At an air-water boundary, some light reflects while transmitted light changes direction.
✨ Pro tip
Want a second refraction trick? Stand a pencil in the glass of water and look from the side. It appears bent or even "broken" at the water's surface. That's refraction too: light from the underwater part bends as it leaves the water, so your eyes are fooled about where the pencil really is.
🔬 Make it a real experiment
Measure the glass-to-card distance where the arrow first reverses, then repeat three times. Compare round glasses with different diameters or fill levels. Predict which setup will have the shortest flip distance, record the measurements and let the evidence decide—the result depends on more than width alone.
What Will You Develop and Learn
- Define refraction and refractive index
- Explain how a cylindrical lens produces a left-to-right reversal
- Distinguish optical behavior from physical mass density
- Change one variable, measure the flip distance and compare trials
Key takeaways
- Refraction occurs when light crosses into a material where its speed is different and its path changes direction.
- Refractive index compares light's speed in a vacuum with its speed in a material.
- Physical density and refractive index are not the same property.
- A round glass of water acts as a cylindrical lens, focusing primarily from side to side.
- At the right distance, rays from opposite sides cross and a sideways arrow appears reversed left to right.
Frequently Asked Questions
What is refraction in simple terms?
Refraction is the change in a light ray's direction when it crosses between materials in which light travels at different speeds. A ray must meet the boundary at an angle to bend; one that enters perpendicular to the surface continues straight.
What is the difference between reflection and refraction?
Reflection sends light back from a surface. Refraction changes the direction of light that passes into a different material. Both can happen at the same boundary: some light reflects from water while some enters and refracts.
Why does light bend when it enters water?
Light travels more slowly in water than in air. When a wavefront reaches the boundary at an angle, one side changes speed first and the path turns. Refractive index measures the speed relationship. This optical property should not be confused with physical mass density.
Why does the arrow appear reversed through a glass of water?
The curved glass and water act together like a cylindrical lens. When the drawing is far enough behind it, rays from the arrow's opposite sides cross on their way to your eyes. Left and right therefore appear exchanged. Close to the glass, the arrow may look magnified or distorted instead of reversed.
Why does a pencil look bent or broken in a glass of water?
The light from the underwater part of the pencil bends as it leaves the water and enters the air before reaching your eyes. Your brain assumes light travels in straight lines, so it "sees" the underwater part shifted, making the pencil look bent at the surface.
What do I need for the refraction experiment?
A round, clear glass, water, paper and a dark marker are enough. A ruler is useful if you want to measure and compare the distance where the arrow reverses.
Why does the arrow flip sideways but not upside down?
A tall round glass is shaped like a cylinder. Its curved sides focus light strongly in the horizontal direction, while the vertical surface profile is nearly straight. It therefore swaps left and right without necessarily swapping top and bottom.
If you liked this experiment and want more cool stuff to do, we recommend you continue your exploration of physics. We bet you would like to learn how to make a potato battery! Or try some amazing balloon experiments!
Happy exploring!
