November 10, 2025, 3:40 pm | Read time: 3 minutes
Just looking at the deep blue of the sea brings relaxation and deceleration for many. It’s not just the vastness that impresses, but also the color of the water. But why is the sea actually blue?
Sometimes it’s storm-tossed and gray, sometimes green like a stagnant garden pond. Dream beaches in the South Seas often lie by turquoise waters. Most of the time, however, the sea presents itself to us in a deep blue color. How does the human perception of the sea’s color come about? Why does an ocean mostly appear blue to us? TRAVELBOOK asked an expert.
“Many people think the sea is blue because the sky is blue and it is reflected by the water. Unfortunately, it’s not that simple,” explains Tobias Kampfrath, professor of experimental physics at the Free University of Berlin. So why is the ocean blue? According to Kampfrath, it mainly has to do with how the water molecules interact with the incoming light.
Professor explains why the sea is blue
“As soon as a sunbeam penetrates the seawater, two processes occur simultaneously: On one hand, the light is absorbed by the water. On the other hand, it is scattered in all directions by the water molecules and can thus reach our eyes,” explains Kampfrath. These two processes ensure that the scattered light contains more blue than red components.
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This is partly because red light is absorbed more strongly than blue light. “As a result, blue light can penetrate deeper into the water and be scattered by more water molecules to us,” says Kampfrath. Additionally, blue light, due to its shorter wavelength, is scattered much more strongly than red light, which has a significantly longer wavelength. This is why the sea appears blue to us.
According to Kampfrath, however, there is a second reason for the intense perception of the color blue besides the light’s impact on the water–namely, the human eye itself. “Our visual organ is much more sensitive to blue and green radiation than to red. That’s why we recognize a blue or green color spectrum much better,” explains Kampfrath.
The color of the sea can change for numerous reasons
But we all know that the sea is unfortunately not always a radiant beautiful blue. We are all familiar with the gray to brownish image that the Baltic Sea presents in bad weather. And sometimes the sea also shows a strong greenish color. Professor Kampfrath explains: “The light conditions just described are especially true when the sea is clean and deep. There can, of course, be numerous circumstances under which our color perception changes.”
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For example, when there is a high proportion of plants and plankton that secrete the green substance chlorophyll, the sea is perceived as greener. Mud and severe weather, on the other hand, can make the sea appear gray and brown. Whereas a white sandy bottom makes the waters of the Caribbean appear brighter.
Sometimes, when the proportion of microorganisms, algae, and brine shrimp that produce the red substance beta-carotene is very high in a body of water, it can also appear reddish or pink, leading to seemingly supernatural images of pink-colored rivers and lakes. So, the color of the sea is by no means just “blue,” but very changeable and dependent on numerous factors.