- Age:5+
- Time:10 min
- Difficulty:Easy
- Mess level:Low
- Supervision:No
🧒 In one sentence
A glass of water bends light like a lens, so a picture held behind it can flip to face the other way.
Reflection and Refraction
Sight is our dominant sense. Most of all information we receive is coming through our vision. That’s huge! But how do we see the world around us? It’s actually not only about our capabilities. Objects must have one important property to be seen: they must reflect light. Light is such a tremendous topic that there is special discipline devoted to studying it - optics. Optics is a part of the physics that studies the light - its properties and the behaviour.
So, let’s talk about two important behaviors of the light: reflection and refraction. Reflection is a familiar concept. You look at any polished surface such as the mirror and you see your own reflection. An image is virtually identical, but it’s reversed in the front-back direction. There are two rays of light in play here, incoming and outgoing. Incoming ray hits the surface under a certain angle and then outgoing ray goes under that same angle in the opposite direction. That’s called Law of reflection.
While passing through more than one medium, the light will be refracted
Refraction, on the other hand, means bending of the light. Bending happens whenever light changes medium in which it’s traveling.
For example, moving from water to air would produce this behavior. That occurs because of the difference in the density of different mediums. Air is in the state of gas and molecules of gas are spread out so there is less chance of light hitting them. But let’s put water into the mix. Here, molecules are more packed and that means light moves slower through them. We measure that slowdown with a number called the refractive index: it tells us how many times slower light travels in a material than it does in a vacuum.
Air, for example, has a refractive index of 1.0003, which means light in air is barely slower than in empty space. Water has 1.333, so light travels about a third slower in water than in a vacuum. The bigger the number, the more the material slows light down. When light moves from the air into the water it slows down, and that is what makes it change direction.
The Science Behind Light Bending
In our experiment, a light went from the air through the water and then back through the air to get to the drawing. We explained that light refracts when it changes medium and what does that mean.
In this case, the water behaves like a magnifying glass. Light bends towards the center, narrowing until it gets to the one point which we call the focal point. From the focal point, light rays switch direction so the left one goes to the right and the right one goes to the left. The result is that our drawing looks reversed!
Two details make or break this trick, and they are worth knowing.
First, the drawing has to be far enough behind the glass - past that focal point. Hold the paper right up against the glass and nothing flips; it just looks a bit magnified. Slide it slowly backwards and, at a certain distance, the image suddenly snaps around. Finding that distance is the experiment.
Second, why does an arrow or the word "OK" work so perfectly? A normal round glass is a cylindrical lens - a tube, tall and straight. It bends light strongly from side to side but not up and down, so it flips things left-to-right only. An arrow pointing sideways dramatically reverses; "OK" turns into its mirror image. A drawing that looks the same left-and-right (like a circle, or the letter "O" on its own) would show no change at all, which is why the props matter.
Materials Needed for the Refraction Demonstration
You will need a glass of water, some drawings or a pencil and the paper
- Transparent glass or bottle
- Water
- Some drawings (or pencil and paper)
Instructions for the Refraction Demonstration
For step-by-step instructions on how to conduct this experiment watch the video on the beginning of the article or continue reading...
- Draw some images you would like to test - a sideways arrow or the word "OK" works great
- Pour water into a glass
- Put the image behind the glass and look at it through the water
- Start with the paper close to the glass, then slowly move it further back. Watch for the moment the image flips - that happens once the paper passes the focal point. What do you see?
What will happen to an arrow drawing when you look at it through the glass of water?
Make your prediction, then tap an answer to check!
✨ 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
Find the exact flip point. Draw an arrow, then move the paper slowly away from the glass and mark with a ruler the distance where it first flips. Now try a fatter glass and a thinner one, filled to the same level, and measure the flip distance for each. A wider glass bends light more strongly, so the flip should happen closer.
What Will You Develop and Learn
- Basics of optics
- What is reflection and Law of reflection
- What is refraction and refractive index
- Scientific method
Key takeaways
- Optics is the part of physics that studies light and how it behaves.
- Reflection bounces light off a surface at an equal angle - that's how mirrors work.
- Refraction is light bending when it passes from one medium into another, like air into water.
- Light bends because it travels at different speeds in different mediums; the refractive index measures that.
- A round glass of water acts like a cylindrical lens, crossing the light rays at a focal point so a drawing held far enough behind it flips left-to-right (which is why a sideways arrow or "OK" is the perfect thing to draw).
Frequently Asked Questions
What is refraction in simple terms?
Refraction is the bending of light as it passes from one material into another - for example from air into water. It happens because light travels at different speeds in different materials, and that change of speed makes the light change direction.
What is the difference between reflection and refraction?
Reflection is light bouncing off a surface, like your image in a mirror. Refraction is light passing through into a new material and bending on the way. A window does both at once: you see through it (refraction) and catch a faint reflection of yourself (reflection).
Why does light bend when it enters water?
Water is denser than air, so light travels more slowly through it. When a light ray crosses from air into water at an angle, one side slows before the other, which pivots the whole ray and changes its direction. The refractive index is the number that measures this speed change.
Why does the arrow appear reversed through a glass of water?
The round glass full of water behaves like a lens. It bends the incoming light rays towards the middle until they cross at the focal point. Past that point the rays swap sides - left becomes right - so the arrow behind the glass looks flipped. The catch: the drawing has to be held far enough back, past the focal point. Right up against the glass it only looks magnified; slide it away and it suddenly flips.
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?
Almost nothing - just a clear glass or bottle, some water, and a drawing or a pencil and paper. Draw an arrow or a word, look at it through the water-filled glass, and watch it reverse.
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!
