Longer-wavelength colors, like red and orange, are easiest for water molecules to absorb, and so they drop out first. The color red becomes invisible to the human eye at around fifty feet down; yellows disappear at around a hundred and fifty feet; greens at two hundred feet, and so on, ultimately leaving only stronger, shorter-wave colors like blue and purple. The blue ocean water (and sky) we see from the surface has nothing to do with the color of water or air — both, of course, are colorless. Tropical water appears intensely bluish-purple because the visibility extends for hundreds of feet,
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