The usual answer to this question is that the water has no color, that it is "clear" or "transparent" and that the sea only appears blue because of the sky reflected in it. It's a delusion. The true color of the water is blue. Incredibly faint shade, but still the water is blue. Moreover, it becomes more blue as the thickness of the observed water sample increases.
You can see for yourself if you look into a deep hole in the snow or through the thick ice of a frozen waterfall. And if you take a very large and very deep white pool filled with water and look through it, the water will turn out to be blue.
The color inherent in pure water can be observed by looking at a white light source through a long tube filled with water and closed at both ends with transparent windows. Visible blue is the result of scattering of the blue portion of the visible spectrum.
White swimming pool with clear water | Abstract photo created by jcomp – www.freepik.com
However, this subtle hue does not explain why water sometimes takes on a striking blue appearance when we look not through it, but at it. The reflected color of the sky definitely plays an important role here. On a cloudy day, for example, the sea doesn't look that particularly blue. In other words, the color of the sea depends on the color of the sky, the number and nature of the clouds, the height of the Sun above the horizon.
But still, not all the light that we see is reflected from the surface of the water; some of it comes from under this surface. The dirtier the water, the more light it reflects.
Large bodies of water, such as seas and lakes, tend to contain high concentrations of microscopic plants and algae in the water. They reflect and scatter the light returning to the surface, resulting in the huge color variety we see. This explains why the Mediterranean sometimes appears emerald green under bright blue skies.
How exactly are green shades formed near the water? An important component that absorbs light in ocean waters is chlorophyll, which is used by phytoplankton to produce carbon through photosynthesis. Due to chlorophyll, phytoplankton absorbs more in the red and blue parts of the spectrum and reflects in green wavelengths. Therefore, sea waters with areas of high concentration of phytoplankton will have a blue-green and green color, depending on the density of the phytoplankton population.
What about the red color of the sea water? Outbreaks of certain types of algae are responsible for the phenomenon of red tide, when the surface of the sea is covered with stripes of red hues.
Maldives | pixabay.com
The same thing happens with ice. Ideal ice is transparent, but any inhomogeneities lead to the absorption and scattering of light and, accordingly, a change in color.
Snow glaciers appear white from a distance, but up close and when shielded from direct ambient light, glaciers generally appear deep blue due to the long path length of internal reflected light.
A relatively small amount of ordinary ice appears white due to the large amount of air bubbles, and also because a small amount of water appears colorless. In glaciers, pressure causes air bubbles to be squeezed out, increasing the density of the ice created. A large amount of water appears blue, so a large piece of compressed ice or a glacier will also be blue.
Blue ice off the southern coast of Greenland | wikimedia.org
But what about the color of the water in the rivers? As a result of erosion, rivers often carry particles of soil or rock in suspension. It is the presence of soil and other solids in the water that largely determines the color of river water. Thus, the water in the rivers is:
- transparent (without color) – as a rule, near mountain and high-mountain rivers, fed by melt water from snowfields or glaciers, flowing in granite or basalt;
- yellow (yellow-red) – near lowland rivers and especially desert rivers that carry a large amount of sand, clay and solid particles of organic origin (for example, the lower reaches of the Tarim in China or the Kuban in Russia);
- dark or black – especially characteristic of rivers flowing in areas with dense tree vegetation (for example, in the jungle). A prime example is the Rio Negro River in Brazil.
- white (white-gray) – in addition to suspended particles, air bubbles give white color to river water when water foams on rapids and in waterfalls. In English there is a phrase "white water" (white water), which means rapids.
In Japan, a certain scale is common in the fishery to describe the pigmentation of water “from carp to trout”, since carp can only be found in muddy water, and trout only in clear water.
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