Water. It’s essential to life on our planet. But did you know how weird water is? In this series we are looking at the peculiarities of H2O. In Part 1, we showed that floating ice makes life possible. In Part 2, we talked about freezing and boiling temperatures. Today we will look at the interaction between water and light.

Water is transparent to visible light. The only way to see anything is for photons to reflect off of something, and then register on the photosensitive cells in the eyes. If water wasn’t transparent, we wouldn’t be able to see anything, at least not in the range of light that we do now.

Water is opaque to U.V. 95% of U.V. radiation is absorbed in the first four inches of water. Because the temperature of an object goes up when it absorbs radiation, this means that the surfaces of Earth’s water bodies are warmed, while subsurface water remains cool. The warm upper layer of water is called the epilimnon.

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