The Alligator’s Secret Mirror: How Total Internal Reflection Connects Aquariums and the Internet
I’m Ken Kuwako, your Science Trainer. Every day is an experiment.
A quick quiz for you: What is this??
It looks like a mirror… but the surface is rippling. Here’s your second hint.
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It’s this!
Something is peeking its face out from above the mirror. You know the answer now, right?
The answer is here.
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Yes, it’s an alligator!
From above, you can clearly see its entire body underwater…
But when you look up from underwater, you can’t see the outside world (above the water). The surface looks exactly like a mirror.
The Water Surface Becomes a “Mirror”? The Light Magic of “Total Internal Reflection”
Let’s look at the alligator again.
From above:
From below (underwater):
The water surface acts like a mirror, and you can’t see the face of the turtle or alligator that should be above the water. Only the reflection of the creatures underwater is visible on the surface.
This is a phenomenon in physics called Total Internal Reflection.
When light travels from water into the air, it doesn’t go straight; it bends (this is called “refraction”). However, when the angle at which you look up from underwater toward the surface becomes too shallow, the light cannot escape into the air. Instead, 100% of it is reflected back into the water.
This is the true nature of Total Internal Reflection. At this point, the water surface acts as a perfect “mirror,” reflecting all the light.
In the alligator’s photo, it’s the same as us looking up at the water surface from underwater. The light from the “outside world” where the alligator’s face and turtle are cannot enter the water, and the surface acts as a mirror reflecting the underwater scene, which is why it looks that way.
Where Else Can We See “Total Internal Reflection”?
I took these photos at iZoo in Izu, but for something closer to home, I recommend the penguin tank at Sumida Aquarium. This tank is massive, allowing you to view the penguins from various angles—from above, or looking up from below. The moment you look up from underneath and the water surface turns into a mirror, making the penguins appear to be swimming in the “sky” (the water surface), you are experiencing the world of Total Internal Reflection!
Is the Internet Possible Thanks to “Total Internal Reflection”?
Actually, this Total Internal Reflection isn’t just a phenomenon for aquariums. The optical fiber that allows us to view the internet right now uses the exact same principle. Optical fiber is a thin strand of glass that delivers light signals at an incredibly high speed. Light travels through this fiber by repeatedly undergoing Total Internal Reflection. Because the light is 100% reflected off the fiber’s walls and travels in a zigzag pattern without leaking out, information can be transmitted quickly over long distances.
The mysterious phenomenon of the alligator vanishing at the water surface and the reason for the high speed of the internet are both connected by the same science: Total Internal Reflection! Science is all around us! Try to look for it!
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