Eli5 Why is water see through?

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My 4 year old asked me and I think it’s a rather good question that I would like to answer so she understands. Thanks 🙏🏻

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It’s a little bit backwards. Life needed to be able to see through water, so it created eyes that could see the light that water was clear to.

That might need some explanation. All things are “clear” to some kinds of light and “opaque” to other light. Like how an X ray can go right through your skin and see your bones. It’s that way for all light, including visible light.

So there was always some wavelength of light that made water “clear”. And some of those wavelengths are the visible light spectrum.

So when life evolved in the ocean, and eyes developed, it was very useful to be able to see the light that could pass through the water. And so you get eyes that can see in the ocean.

Edit: so the phrase I’d use for the actual 4 y/o is “It’s see-through because eyes were specially made to see through water” or if you want it to sound more awesome but less helpful, “because your eyes are like x-ray goggles for water”

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