From Umbrella Bags to Magic Threads! The Secret of Optical Fibers Revealed in an Instant — Total Internal Reflection Experiment

I am Ken Kuwako, your science trainer. Every day is an experiment!

In my last post, we used a simple umbrella bag to explore “light scattering”—the reason why the sky is blue and sunsets are red. Today, let’s dive into the secrets of fiber optics, the massive infrastructure that powers our modern world!

What exactly is “Fiber Optics”?
What comes to mind when you hear the term “fiber optics”? When I ask my students, they usually tilt their heads and look a bit puzzled. That’s fair—fiber optics are usually hidden away where we can’t see them. But when I show them the real thing and shine a light through one end, watching the other end burst into a bright glow, their faces light up.

“Oh! It’s like those sparkly Christmas tree decorations!”

Exactly! These days, fiber optics are very common as decorative illumination. However, their true power goes far beyond just looking pretty. They are the lifelines of modern society, enabling ultra-high-speed internet and allowing doctors to see inside the body using endoscopes (like gastric cameras). The secret behind this technology is a phenomenon called “total internal reflection,” which traps light inside without letting a single drop escape.

This “magic” might sound complicated, but we can recreate it ourselves using nothing more than an umbrella bag!

Science Recipe: The Umbrella Bag Fiber Optic!
What you’ll need:

• A thick straw or an umbrella bag (the more transparent, the better)
• Soap (liquid hand soap works great, but dish soap or bath salts also work)
• Water
• An LED light
• A laser pointer (the star of the show!)

Instructions:

Fill the umbrella bag with plenty of water. Then, dissolve “just a tiny bit” of soap into it. Start with 1 to 3 drops. Adding too much is the quickest way to fail! Adjusting bit by bit is the hallmark of a true scientist.

Why add soap? It’s to suspend microscopic soap particles (colloids) in the water. When the laser hits these particles, the light scatters in all directions. This is known as the “Tyndall effect,” and it’s what allows us to clearly see the “path of light.”

Make the room as dark as possible. This is where the fun begins!

Gently aim your laser pointer into the end of the umbrella bag.

Check out this video to see it in action:

When you aim the laser into the end of the bag—even at an angle—the light doesn’t escape through the sides. Instead, it bounces off the walls repeatedly until it reaches the other end. As a result, the entire bag glows a brilliant red, like a long, glowing gemstone.

Note: If the soapy water is too cloudy, the light will be blocked and won’t reach the end. Finding that perfect “dilution” is the key to success.

Now for the main event! Try slowly changing the angle of your laser.

As you aim the light at a shallow angle against the wall of the bag… at a certain point, the light will stop leaking out entirely. You’ll see it zigzagging perfectly down the bag!

This is the moment of scientific magic: Total Internal Reflection!

When light tries to pass from water into air, if the angle is shallow enough, the boundary acts just like a mirror and reflects all the light back inside. Real fiber optic cables are precision-engineered to keep this reflection going indefinitely, allowing information to travel for many miles.

Conversely, if the angle is too steep (closer to perpendicular), the light will pass right through the wall and escape. This is called “refraction.”

Learning Through Experience
Before diving into textbook diagrams or solving formulas, I want you to experience that “feeling of controlling and trapping light” for yourself. Witnessing a phenomenon with your own eyes is often the spark that ignites a lifelong passion for physics.

For this experiment, make sure to use a clear umbrella bag (like the ones found at supermarkets on rainy days—just be sure to ask for permission first!). If you want to try this on a larger scale, you can pick up a bulk pack of bags from a packaging specialist.

For those who want to dive deeper, I’ve explained “total internal reflection” and “critical angles” in this video lesson. Check it out!

Why do diamonds sparkle so brilliantly? Why is it hard to see the sky clearly from underwater? All of these everyday mysteries are connected through total internal reflection. Why not take a look at the world through the eyes of a scientist?

Inquiries and Requests
Let’s make the wonders of science a part of your everyday life! I’ve put together many fun and easy experiments you can do at home. Feel free to explore!

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