Water as Optical Fiber

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Let’s try an optical fiber experiment using flowing water.
• Water flows out through a hole drilled in the side of a plastic bottle. At the same time, a laser pointer is directed from behind onto the stream of water flowing out of the hole. When you do this, you will observe the light bending and following the flow of the water. (You can also try using a flashlight instead of a laser pointer. Please be careful to avoid light leaking into the surrounding area during the experiment.)
• When light travels through water, which has a higher refractive index than air, it undergoes total internal reflection at the boundary between water and air. This causes the light to remain inside the water stream without escaping. As explained in the “Principles of Optical Fiber,” total internal reflection occurs when the angle between the light and the water surface is small. Even if the water flow bends slightly, the light continues to reflect internally and follow the flow.
• This phenomenon is based on the same principle as optical fibers.
• Use paint or a similar substance to lightly color the water. Since clear water is transparent, it can be difficult to see how the light passes through it. Adding color makes the light path visible.
• In this experiment, I also applied black vinyl tape around the hole in the side of the plastic bottle. This prevents light from leaking out of unintended areas, which could illuminate the flowing water unevenly and interfere with proper observation.
• This video was produced with the support of the JSPS Grant-in-Aid for Scientific Research (18K03956).

[Keywords]	Total internal reflection
[Related items]	Principles of Optical Fiber, The Height of Water and Its Energy
[References]	Ryozo Ishiwata and Mitsumasa Nemoto, “The Wonder of Flow,” Kodansha Bluebacks, pp. 98–101.

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