The oceans appear blue from the surface because water selectively absorbs wavelengths of other colors in the light spectrum, leaving only blue light to be reflected and reach our eyes.
When the sun sends its light to the surface of the sea, it is quite an adventure for each ray of light. First, it is important to know that the white light we see is actually made up of several different colors (like when you use a prism to create a small rainbow). In water, these colors do not propagate in the same way: some penetrate deeper, while others are quickly stopped or absorbed. The blue and green fare better than the other colors, which is why we see them more. In contrast, warm colors like red or orange quickly disappear as one descends underwater. This subtle interplay between the natural characteristics of water and the different light colors is at the heart of this visual phenomenon.
When sunlight enters the water, some of the light rays are scattered in all directions by water molecules and suspended particles: this is known as Rayleigh scattering. This phenomenon is particularly intense for short wavelengths, such as blue or violet, unlike reds or yellows which pass through with less disturbance. As a result, blue hues are more widely dispersed all around us, giving the ocean that bluish appearance when viewed from the surface. In fact, it’s the exact same process that explains why the sky appears blue, except that here, it’s the air molecules that do all the work.
Seawater absorbs certain colors better than others, and this phenomenon partly explains its bluish appearance. Warm colors like red or orange are quickly absorbed within the first few meters below the surface. As you go deeper, it is soon the turn of yellow and green shades to gradually disappear. As a result, only blue tones manage to penetrate deeply and are easily reflected back to our eyes. The shallower the water, the clearer it will be, revealing turquoise or blue-green hues, typical signatures of shallow regions or sandy bottoms. In contrast, deeper water absorbs almost everything except dark blue, which creates the characteristic intense blue effect of the deep oceans when viewed from the surface.
The ocean is teeming with life and contains many small floating particles. For example, phytoplankton, these tiny marine algae, can sometimes give the water a green, bluish, or turquoise hue, depending on their species and abundance. Some very clear tropical waters often owe their beautiful turquoise color to tiny calcareous fragments (calcium carbonate) suspended in them. Mineral particles, sediments, sand, or mud carried by rivers and stirred up by waves also significantly influence the color of the water, giving it a somewhat duller, brownish, or greenish appearance. The more matter or organisms there are in the water, the more the colors will be altered from the deep blue typical of the purest and clearest waters.
In depth, the blue hues of the ocean gradually fade away, giving way to total darkness. At depths of 200 meters, sunlight struggles to penetrate, creating an area known as the 'twilight zone.'
The turquoise color of tropical lagoons mainly comes from the shallow depth of the water and the seabed made up of white sand or light-colored corals, which reflect more sunlight.
Some lakes or bodies of water sometimes display pink hues! This astonishing phenomenon is due to the presence and high concentrations of microscopic organisms such as the algae Dunaliella salina, which produce a bright pink coloration under high salinity conditions.
The phenomenon of red tides has nothing to do with the tide itself but rather refers to an excessive proliferation (bloom) of toxic microalgae. These algal blooms can be harmful to the marine environment and human health.
Offshore, the ocean tends to be deeper and generally contains less suspended sediments and organic matter. These conditions enhance light absorption, giving the water a darker or more intense blue color. In contrast, coastal areas are often shallower and laden with sediments that alter the perceived coloration.
No, the color of the oceans varies depending on several factors such as depth, the type and amount of living organisms, and suspended particles. Thus, certain parts of the ocean appear green or even brown, especially near coastlines or estuaries.
Algae, and more specifically phytoplankton, contain pigments that alter the apparent color of the water. When they are abundant, these algae can give the sea a greenish or even reddish hue during specific phenomena such as red tides.
Not necessarily. Although clear, bluish water often indicates a low concentration of particles or algae, it does not necessarily guarantee the absence of chemical pollutants. Conversely, murky water is not always synonymous with pollution.
Water in small quantities appears transparent because light passes through easily without significant absorption or scattering. In contrast, when it is present in large quantities, such as in the oceans, the combined effect of selective absorption and light scattering enhances the blue color perceived from the surface.
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