Rainy Days Welcome! Make a Rainbow in Your Classroom — How a Plastic Bottle Becomes a Magic Tool for “Light Dispersion”!
I’m Ken Kuwako, your Science Trainer. Every day is an experiment.
When teaching the properties of light, especially the concept of light dispersion, how do you spark your students’ curiosity? You can explain, “Light can split into lots of different colors,” but wouldn’t it be much more exciting to actually show them the phenomenon?
As you probably know, on a sunny day, you can create a beautiful rainbow simply by spraying water into the air with a garden hose. Stand with your back to the Sun, let the sunlight interact with the sparkling droplets, and suddenly a vivid seven-colored arc appears. It really is a magical sight! I once tried making a rainbow with my students in the schoolyard, and I got so caught up in how beautiful it was that it was surprisingly difficult to get everyone back to the lesson afterward. (Haha!) And, of course, there’s one big drawback: you’re at the mercy of the weather, so you can’t exactly do it whenever you want.
That’s why I’d like to introduce an experiment that lets you recreate a surprisingly vivid rainbow, easily and indoors, no matter what the weather is doing outside! The setup is simple, and every student can experience the beauty of light dispersion with their own eyes. So, let’s take a look at this magical little “science recipe”!
Science Recipe: Turn an Ordinary Plastic Bottle into a “Rainbow Machine”!
The star of this experiment is, believe it or not, a 1.5-liter round plastic bottle! Yes, the kind of cylindrical bottle often used for drinks such as Mitsuya Cider can become a surprisingly effective tool for showing off the beauty of light dispersion.
【What You’ll Need】
- A 1.5-liter round plastic bottle (a smooth-sided bottle, such as one used for Mitsuya Cider, works especially well)
- Water
- A white light (an LED light is especially recommended. As we’ll see later, brightness really matters!)
【How to Do the Experiment】
1. Fill the plastic bottle with water:
First, fill the round plastic bottle completely with water. Pour slowly so that you don’t introduce too many air bubbles.
2. Darken the room and shine the light:
Make the room as dark as possible. Close the curtains and turn off the lights.
Next, shine the white light at the plastic bottle. The key is to stand with your back to the wall and shine the light toward the bottle. The light reflected inside the bottle should then be projected onto the wall behind you.
3. Adjust the angle of the light and observe:
Slowly change the angle of the light while carefully watching the light projected onto the wall. And then… there it is! A brilliant seven-colored rainbow should appear right before your eyes!
With this method, all you need is one plastic bottle and one light for each group, and every student can easily enjoy the rainbow for themselves. When you actually try it, you’ll find that it looks far more beautiful than it does in photographs. It’s the kind of sight that makes you instinctively go, “Wow!” And perhaps best of all for teachers, the entire experiment can be done right in the classroom, so once the rainbow disappears, you can get straight back to the lesson!
A 100-Yen-Shop Light Can Make a Huge Difference!
By the way, the light I use for this experiment actually came from a 100-yen shop. I had been using a different light before, but the new one I used this time was much brighter and produced a clearer, more vivid rainbow.
Honestly, I found myself thinking, “Why didn’t I use this one from the start?” (Haha!) It was a great reminder of just how much the brightness of the light can change what you see.
The Science Behind the Rainbow: Understanding Light Dispersion
The beautiful rainbow produced in this experiment can be explained by a scientific phenomenon called “dispersion of light.”
Sunlight and white light from a lamp are actually made up of many different colors of light mixed together, including red, orange, yellow, green, blue, indigo, and violet. When these different colors enter a water droplet—or, in this experiment, the water inside the plastic bottle—each color travels through the water at a slightly different speed. As a result, each color is refracted by a slightly different amount.
As shown in the diagram above:
- Red light bends less than the other colors (it has a lower refractive index) as it travels through water. Because it follows a relatively straighter path, it appears on the outer edge of the rainbow.
- On the other hand, violet light bends more (it has a higher refractive index) in water, so it is refracted more strongly and appears on the inner edge of the rainbow.
In this way, when light is refracted and reflected at slightly different angles according to its color, the individual colors separate from one another and eventually reach our eyes as a beautiful seven-colored rainbow.
This experiment is simple enough for elementary school students to enjoy, yet it hides some wonderfully deep physics involving refraction and dispersion. That combination of simplicity and scientific depth is what makes it such a great experiment.
What other experiments do you use to demonstrate light waves in your science classes? I’d love to hear how you put them to use!
And next time, let’s use this same plastic bottle to recreate a “sunset”! Stay tuned!
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