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Question: Why does deep water appear to be blue, when water is clear?
Answer: The apparent color of water is a combination of the scattering of incoming sunlight (solar radiation) and absorption by the water. Solar radiation includes a wide range of wavelengths, some of which we recognize as colors (red, orange, yellow, green, blue, violet). When sunlight intercepts the surface of a body of water, some is reflected off the surface, and some penetrates into the water. Water absorbs some of that, preferentially absorbing wavelengths in the red range, which results in us perceiving the color as a net blue color (the complementary color). That helps explain why ice in a glacier is blue, for example. Some of that incoming light energy also is scattered by waves and ripples and by particles in the water. And some chemicals and organisms in the water can give color as well (such as red algae helping color the Red Sea or dinoflagellates coloring a "red tide"). So the color of the water is a combination of scattering, absorption, and the incoming radiation itself. If the water is deep, the surface is smooth, the water is clear (i.e. little suspended or dissolved solids), and the
sky is clear, you can see a deep blue or turquoise. But change any of these, and the color will change--grey water on a cloudy day, greenish water if there's a lot of very fine sediment (or brown if there's a huge amount!), light blue if the water is shallow.
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