Why is seawater blue?

Ever wonder why the sea is - when not green - blue? After all, water should be transparent, right? But, in fact, there are some hypotheses to answer this question related to physics, the way light behaves, and the composition of the oceans.

Seawater can actually come in many colors, depending on the particles that are suspended in it, the amount of sunlight falling on it, and the depth. In addition, the colors we see depend on the wavelengths of light that water reflects and are visible to our eyes. This is where physics comes in ...

Optics!

Depending on the composition of a particular material, the different lengths of light travel through it differently. In the case of seawater, the wavelength near the blue spectrum travels to greater depths, while the lighter wavelengths - reds, yellows and oranges - are absorbed faster.

In addition, water molecules help propagate blue wavelengths by absorbing lighter wavelengths and reflecting shades of blue in many directions. This is why we perceive the color of the oceans as blue. But what about seas with green or even brown waters?

Colors

In the case of the green sea, there may be a large abundance of sediment or marine plants in the water, interfering with the way light is reflected. In the case of brown waters, which usually stay like this after a storm has passed, the color is due to the action of the wind and the more agitated currents, which collect more sand and sediment, which, in turn, are suspended in the water.

Finally, another hypothesis is that the water reflects the blue sky, although this effect only occurs when the sea is very calm and at relatively low angles. However, most oceans are completely dark, because very little light can reach depths greater than 200 meters and no light penetrates more than a thousand meters. And do you, reader, know any more reason why the waters of the seas are blue?

* Originally posted on 06/26/2013.

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