Why is the sea blue?

When observing a satellite photograph of the Earth, it can be seen that most of it is covered with sea, with waters that in areas seem to be a very dark blue color, in others a bit lighter, and in certain regions even a greenish tone. But when we visit these seas in person and hold a little of their water in our hands, we notice that it is transparent (maybe not perfectly) but definitely not blue, and yet, when looking at the sea from a distance, it still looks blue. Why is this? What color is the sea really? Is it just an optical illusion? And why can the seas be seen in different shades?

Colors

To understand the reason for color, first we need to understand how light works.

Objects are of a certain color due to the amount of light that they can absorb, since light is composed of the sum of all the colors that make up the visible spectrum. Each color corresponds to a certain wavelength, in such a way that materials are capable of absorbing some of them, none or even all of them, and depending on this they will reflect the colors that correspond to the wavelengths that they cannot absorb.

Thus, for us to see the sea as blue, it must absorb long wavelengths, which correspond to red tones. And practically that is what happens; but it is not as simple as it is with other objects.

Colorless water?

Water is made up of hydrogen and oxygen, colorless substances, and consequently, water is also considered colorless. And even though the water of the seas is not in its pure state, since it coexists in the depths with many other substances, it is still water; water that, in small amounts, seems to have no color at all.

The explanation for why seawater appears to have color is similar to the one for why the sky is blue, but not the same: in the sea, the main player is absorption. Water molecules absorb long-wavelength light (red, orange and yellow) most strongly and blue light very little; like a filter, they leave the blue tones behind (NOAA). The blue light that survives is scattered in all directions as it collides with water molecules and suspended particles, and some of it returns to our eyes. This scattering is related to Rayleigh scattering, which occurs when light collides with particles smaller than its wavelength and affects short wavelengths, such as blue, the most.

Water molecules mainly absorb long wavelengths (red, orange and yellow) and far fewer of those that cause bluish tones, regardless of whether it is seawater or bottled water; despite looking transparent, water does filter light selectively. The difference between one and the other is the amount of molecules that light meets on its way, since the absorption of red light and the scattering of blue light increase the thicker the layer of water it passes through; so a bottle of water does not contain enough water for the effect to be visible.

This same thickness factor causes significant differences in the tonality of seawater, since the greater the depth, the more intense the blue color of the sea.

Other factors

In addition to the absorption and scattering of light, there are many factors that cause differences in the tones of the seas, such as pollution, the angle of incidence of the sun throughout the day, the presence of certain species of algae that can even cause pink tones in the water, among others.

One of the most influential factors in the coloration of the seas is phytoplankton, since these marine microorganisms contain chlorophyll, which is responsible for giving the sea its green tones. In this way, the sea in a region does not always look the same shade, because like any other species, the population of phytoplankton can decrease, increase or migrate to other areas, causing significant variations in the color of seawater.

There is no doubt that the sea is one of the most impressive natural wonders on the planet. Understanding the phenomena that make it so special is almost as impressive as knowing its flora and fauna, but that is another story to tell in another scientific whim.