Why is the ocean blue? The simple scientific explanation!

La mer sous le ciel bleu

It is indisputable, the sea is blue in its natural landscape. But when you put it in a container, like a bottle, it becomes transparent. Here are the explanations according to the fundamental laws of physics and optics!

Sometimes it is tempting to think that the blue color of the sea is the reflection of the blue sky stretching above her. Also, this color is not constant, we can see empirically that it changes depending on environment and weather. For example, when the weather is cloudy and rough, the ocean loses its bluish color and appears to turn gray… There are even green seas and red seas… But why ? In truth, he is a simple optical phenomenon. To understand it, however, it will be necessary to recall some basic physical notions.

Another reason, in fact more important, is the dispersion of photons of energy by solar radiation. This is what essentially contributes to making the sea blue.

solar radiation

The sun is a nuclear fusion star. Spread colossal energy in the form of electromagnetic waves that reaches us Is named solar radiation. These waves can be visible or invisible to the human eye.

The ionosphere and atmosphere reflect part of the radiationwhile others come to the surface clouds, oceans or continents. They can be classified into two categories : short waves and long waves.

short wave rays

Shortwave lightning is extremely damaging. Fortunately, the atmosphere and the ozone layer they constitute true shields that prevent them from harming life. They include in particular:

  • X-rays
  • beta rays
  • UVC rays
  • Gamma rays

long wave rays

the long wave rays They are the ones that reach the surface of the earth. These are mainly:

  • Ultraviolet
  • visible light
  • infrared

The visible light spectrum

Various colors make up natural white light, which corresponds to a well-defined range of light and frequency.

In order to explain the color of the sea, it is necessary to be interested in the visible spectrummore precisely between frequencies 380 to 780 nanometers (nm). represents approximately 39% light reaching the earth’s surface.

We could all admire the dazzling color of a rainbow. Well, these colors exactly match the spectrum visible to the human eye. They are generally seven in number, including red, yellow, green, blue, violet, orange, and indigo.

A rainbow and airy plants

a part is reflected according to the albedo of the struck surface. Another part will be absorbed in the form of heat. another one is exploited by living organismslike photosynthesizing plants.


Is here amount of reflected light depending on the nature of the affected material. The more reflective a surface is, the higher its albedo. For example, that of fresh snow is 0.87, which means that 87% of solar energy is reflected by this type of snow.

In the case of the sea, the angle of incidence of the sun plays a considerable role in the intensity of its albedo. When it is low on the horizon, its albedo is around 15%. It can reach 60% when the sun is higher.

Have an impact on the color of the sea. In the polar areas, in fact, the sea is greenish blue, while in the tropical and equatorial seas it turns indigo blue.

But where does the blue color of the sea come from? ?

absorption and reflection

For a long time it has been thought that the blue color of the sea comes from the reflection of the sky. That’s right, it can help color seawater, but only in a small part.

Another reason, in fact more important, is the dispersion of photons of energy by solar radiation. This is what essentially contributes to making the sea blue.

The reds, yellows and oranges represent lower energy photons. The molecules present in the water absorb them easily.

Nevertheless, the most energetic of the spectrum correspond to violet and blue. they are much more hard to absorb. They are reflected by water moleculesattributing to the sea its appearance as we see it.

Water weakly absorbs wavelengths. Therefore, large amounts are needed for the blue it shows.

LOOK ALSO: Why is the sea salty?

The role of depth

As we go deeper, the number of suspended particles increases. This has the consequence of reflecting the color of the unabsorbed spectra which, in this case, is the blue.

That is why the water in a bottle is transparent, the water in a swimming pool is slightly blue and finally the water in the sea. deep blue.

This is a depth-absorption relationship of some colors of the spectrum of light in the ocean.

  • The Red : will be the first to be absorbed. You can’t see beyond 25m.
  • Yellow : goes a little further than about 100m depth.
  • The green : can go up to 250m.
  • Blue and purple: have wavelengths that allow them to penetrate to greater depths, they are perceptible up to 500m.

temporary color changes

The weather can also influence the color of the sea, sometimes you can see that the sea is grayish when the weather is less clear. The churning of the water due to the storm. disturbs the reflection of colors. And the wind, sometimes violent, photon scattering impacts. They shake and suspend.

Grayish sea in rough weather

On the other hand, if the water is calm and transparent, background color closing can affect the apparent color of the water. This is why, for example, the sand makes the sea appear yellowish gray.

the other colors of the sea

the green sea

In circles phytoplankton friendly photosynthetic like algae for example, it happens that the waters take on a green color. This is because chlorophyll absorbs blue waves faster. In this configuration, green becomes the only visible wavelength that is not absorbed.

A green sea with a turtle

It is a phenomenon that has become more and more frequent in recent years on the French coast: sea green, and even frankly fluorescent green. This would be related in particular to global warming, which would favor the proliferation of microalgae lepidodinium chlorophore.

the red sea

For several years, we can observe a change in the color of the water in the Atlantic Coast, more specifically, along the southern coast of Brittany. During this change of season, the water turns red – brown. This phenomenon is caused by the rapid multiplication of an algae I call Lingulodinium Polyhedra.

A blood red sea

From the phytoplankton family, these species are also bioluminescentthat is, they cast a light blue in color This spectacle is striking when seen at night. This massive proliferation results from abundance of nutrient intake and when the temperature is favourable, they grow fast.

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