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Have you ever wondered why a straw seems to bend when you put it in a glass of water? It’s not just an optical illusion; it’s a fascinating phenomenon called refraction.
This experiment demonstrates refraction through water. When you view the arrow through air, it is pointing in one direction. When you view the same arrow through water, the arrow seems to switch, pointing in the opposite direction!
Let’s talk all things refraction!

How to make the Water’s Trick Refraction STEM experiment
Supplies you will need
For this experiment, you’ll need:
- Printer paper (slightly thicker paper)
- Black marker
- Glass of water

Before you start
If your child struggles with right and left, do a quick lesson in directions before you begin the experiment. What direction is the arrow pointing when you draw it? What direction does it change to when you place it behind water?
Instructions
Here is how to do this experiment with your child:
Step 1: Draw an arrow on the paper with the marker
Using the black marker, draw a thick arrow on the paper.
Place the piece of paper up against something so it can stay upright, like in a clipboard or tape it to a wall.

Step 2: Place a glass of water in front
Place a full glass of water in front of your arrow.


Step 3: Observe
What happens to the arrow when you view it behind the glass? When you pick it back up to view it?
The STEM behind the Water’s Trick Refraction STEM experiment
This experiment teaches:
- Refraction
- Snell’s Law
- Refraction in nature
How it works
This experiment works thanks to refraction. Refraction is the bending of light as it passes from air into water, or vice versa. This bending occurs because light travels at different speeds in different mediums.
When light enters a denser medium like water from a less dense medium like air, it slows down. This change in speed causes the light rays to bend.
In this experiment, we are looking at an arrow drawn on a sheet of paper through a denser medium, water. When we view the arrow through the water (instead of the air), it causes the light rays to bend and the arrow appears to point in the opposite direction.
Refraction
Refraction is the bending of a wave as it passes from one medium to another. Those waves bend because they change speed when they move from one substance to another substance.
Here are some examples:
- Light: When light passes from air to water, it bends because it slows down in the water. That’s why, when you look at the straw from the side of the glass, the straw appears bent!
- Sound: Sound waves also bend as they travel from one medium to another. For example, sound travels faster in warm air than in cold air. This can cause sound waves to bend upwards or downwards, depending on the temperature gradient. That’s why you might not hear sounds that are far away on a hot day.
Why does refraction happen? The speed of a wave depends on the properties of the medium it’s traveling through. When a wave enters a new medium with different properties, its speed changes. This change in speed causes the wave to bend.
Snell’s Law
Imagine a ball rolling from a sidewalk onto a grassy field. The ball will change its direction as it moves from the smooth sidewalk to the rough grass. This is similar to what happens to light when it travels from one material to another,like air to water.
Snell’s law is like a rule that tells us how much the light will change direction. It depends on the materials involved and the angle at which the light hits the boundary between them.
- If the light goes from a material where it moves fast (like air) to one where it moves slower (like water), it bends towards the normal. Think of the normal as a line perpendicular to the boundary between the air and water.
- If the light goes from a material where it moves slowly to one where it moves faster, it bends away from the normal.
So, Snell’s law helps us understand how light behaves when it moves from one place to another, just like the ball changes direction when it goes from the sidewalk to the grass.
Refraction in nature
If you look closely enough, you will see a lot of refraction out in nature. Here are just a few examples:
Mirages: These optical illusions occur when light passes through layers of air with different temperatures. The cooler air near the ground has a higher refractive index than the warmer air above it. This causes the light to bend, creating the illusion of a distant object, like a lake or a road, reflecting light.
Rainbows: Rainbows are formed when sunlight passes through water droplets. The different colors of light refract at slightly different angles, creating the colorful spectrum.
Distorted appearance of objects underwater: When you look at an object underwater, it appears closer and larger than it actually is. This is because light travels slower in water than in air, causing it to bend as it enters and exits the water.
The twinkling of stars: The twinkling of stars is caused by the refraction of light as it passes through Earth’s atmosphere. The atmosphere is constantly changing, causing the light to bend in different directions, making the stars appear to twinkle.
The appearance of a stick bent at the water’s surface: When you place a stick in water, it appears to bend at the point where it enters the water. This is because light travels faster in air than in water, causing it to bend as it enters the water.
More experiments about optics to try out with your child
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