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Why is the Sea Blue?

· Ocean,Sea,Science

Why is the Sea Blue?

As written beautifully by our favorite University of Washington Research Engineer Bill Beaty:

Why is the sea blue?

Very simple answer: pure water is a blue-colored chemical.

Water is blue in the same way that paints and dyes are blue. Blue dye absorbs red light and passes blue light. Blue paint does the same. Pure H2O does the same: water is a blue-colored liquid.

But water is slightly weird. Water has a very, very dilute blue color. Water is like glass, where thin layers appear totally colorless. (You can see the actual color of glass: just look into the edge of a glass plate. Thin layers of ink also appear colorless, but they have to be incredibly thin layers!)

To see the color of water, first we need a layer of water that’s more than 2ft thick. Your bathtub at home is too shallow. To see water’s blue color, it takes a meter-deep backyard pool (a pool made of pure white plastic.) Also, as with blue dye, the thicker the layer, the bluer it looks, until finally it looks blue-black, if the layer is thick enough.

The blue color of water becomes very obvious if we view it against a pure white background. Tropical beaches are best for this, where a layer of water is seen against a submerged background of white sand. The deeper the water, the thicker the layer, the more blue it appears. If you could peel the water off the landscape like jelly, you’d end up with a transparent bright blue layer, with the white sand left behind.

Or, find some photos of indoor swimming pools where the inside of the pool is painted white. Best are photos of white-painted underwater steps, where each step looks more blue, since each step has a thicker layer of blue water above it.


The blue color of water was once a bit controversial. Back in the 1920s-30s some physics books insisted that water was blue because of scattering. In other words, the authors believed that, if you dive down deep and look upwards, the sun would look red. They thought that, deep underwater, everything would have a dull red-orange look, same as sunsets. Of course this was back before the invention of Scuba breathing gear and popular widespread experience with underwater conditions. Today everyone can see that the color of water isn’t caused by scattering, because deep under water, everything looks blue rather than red-orange. Yet some authors are still taken in by those older books, and will try to convince you that the blue of water is caused by scattering similar to the blue of opals, or the blue of “the sky.” Nope, wrong. Crater lake is brilliant blue because water, like blue dye, is a blue-colored chemical.


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