The sea is not red, nor yellow, nor violet: it is blue. Everyone who has stepped foot on a beach can attest to that truth. Yet, water is transparent… How can a clear liquid give rise to a blue ocean? Sometimes, evidences give rise to complex scientific explanations.
Indeed, if the Earth is called the “Blue Planet”, it is not a coincidence: it is covered with 71% water of a beautiful blue color clearly visible from space. Where does this blue color come from? There are two reasons. The first, which seems the most obvious but not the most important, is the reflection on the surface of the sea of diffuse light from the sky, responsible for the blue color of it. This is why the sea is bluer in good weather, when the sky itself is blue, and it is rather gray in bad weather, when the sky is also gray.
However, speleologists have found that even in caves, protected from the sky, the water is still blue if it is illuminated with a white light. Indeed, when a white light, ie solar radiation, hits the surface of the water, a part is reflected and the other is refracted and enters the water.
However, white light is in fact made up of several wavelengths, each responsible for a color of the visible spectrum. This is why light refracted by raindrops forms a multicolored rainbow. Each wavelength (color) is not absorbed as quickly by the water molecules.
These are the red and yellow waves that disappear first, between 10 and 30 meters deep. Around 60 meters, the green disappears in turn and all that remains is the blue, which ends up being absorbed at 90 meters deep. Therefore, the color that is most refracted by the sea is blue, which explains its color.
Sometimes, when the waters are rich in photosynthetic phytoplankton, the blue waves are absorbed more quickly by chlorophyll and the sea takes a green shade, because it is the only visible wavelength that remains.
The color of the sea is of great interest to oceanographers. Since 1978, NASA has been scrutinizing the light emitted by the oceans. These measurements provide information on the content of organic elements and other suspended materials in the surface layers. They make it possible to know the interaction of the wind and the currents with the biology of the ocean and especially to get an idea of the influence of the human activity on the health of the oceans.