The Physics Classroom Hidden in a Pool: What Waves Teach Us About Reflection, Interference, and Huygens’ Principle
I’m science trainer Ken Kuwako. Every day is an experiment.
Take a look into a swimming pool, and you’ll find something far more exciting than a simple splash. Hidden there is “living physics” — the kind you don’t usually see in textbooks. Reflection, diffraction, interference… these words may sound difficult, but all of these phenomena are quietly unfolding inside a single ripple spreading across the water.
Over the past year, I’ve been filming fascinating wave patterns in pools little by little, and I’d love to share some of them with you today. There’s a wonderful DVD called “Mathematical Curves Hidden in Everyday Life,” and while watching pools, I realized that waves themselves are full of surprising beauty and patterns. Once you start noticing them, the surface of the water becomes an entire physics laboratory.
What Happens When You Create Waves in a D-Shaped Pool?
First, here’s a video showing what happens when a wooden ball is dropped into a D-shaped pool to create waves.
You can observe waves reflecting off both a curved surface and a straight surface.
When waves hit the curved wall, they behave much like light reflecting from a concave mirror, converging toward a single focal point. Meanwhile, waves bouncing off the straight wall reflect at the same angle at which they arrived — exactly like the law of reflection for light.
That’s what makes the D-shaped pool so cleverly designed: it allows you to observe two completely different types of reflection side by side.
Line Waves and Huygens’ Principle
Next, let’s look at line-shaped waves. According to Huygens’ principle, these can be thought of as countless point wave sources lined up together.
Huygens’ principle was proposed by the 17th-century Dutch scientist Christiaan Huygens as a way to explain how waves propagate.
The idea is beautifully simple: every point along a wavefront acts as a tiny source that emits its own new wavelet. These countless little waves overlap with one another to form the next wavefront.
When you watch a line wave spread across the water, it almost feels as though thousands of tiny wave generators are standing shoulder to shoulder, launching waves in perfect synchronization.
It’s hard not to admire the sheer brilliance of Huygens for imagining this principle more than 300 years ago.
Ripple Patterns Near a Waterfall That Look Like a Diffraction Grating
Finally, here’s a fascinating pattern that appears near a waterfall. If you look carefully, you can spot stripes that resemble a diffraction grating.
A diffraction grating is an optical device covered with extremely fine grooves that separate light into different colors, much like a prism. The rainbow shimmer on the underside of CDs and DVDs comes from this exact effect.
As falling water strikes the pool surface, waves are generated at regular intervals. These waves overlap and interfere with one another, creating beautiful striped interference patterns.
Whether it’s light or water, interference appears whenever waves interact. There’s something thrilling about seeing the same physical laws repeat themselves throughout nature.
Physics Is Everywhere in Everyday Life
Try searching for physics in your own daily life!
I’ve also created many simulation materials related to Huygens’ principle, so feel free to explore those as well.
Pools, waterfalls, puddles after the rain — waves are always around us. Once you start looking at the world through the eyes of physics, even the most familiar scenery begins to feel completely new.
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