Five-yen Coin Lens

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Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Dip a five-yen coin (a coin with a hole in the center) in water. When you take it out, a water film forms across the hole. Because of surface tension, the water film is maintained even when the surface is curved.
• With more water, the film is thicker at the center, forming a convex lens.
• With less water, the film is thinner at the center, forming a concave lens.
• In the liquid, molecules are mutually attracted (cohesive force), while the water surface acts like a rubber film. The force that tries to pull the film apart is called “surface tension.”

[Keywords]	surface tension
[Reference]	“The Wonders of Flow” Japan Society of Mechanical Engineering, Kodahsha Blue Backs pp. 62-67