Why is the colour of the sea blue?

Usually, it is thought that the sea has its characteristic blue colour since it reflects the colour of the sea. Even if this statement is at some point true, it isn’t the only nor the main reason. The answer to this question is found in the way water molecules affect light.

The seven colours of the visible light

Visible light, which is the type of light that we can perceive, is composed of seven different colours, which can be seen when a rainbow appears in the sky. Light is a type of energy that travels in the form of waves through space. Each colour of the light has a different wavelength. The wavelength is the space there is between a peak and another of each type of light wave.

The greatest the wavelength, the least energy it contains. The orange and red colours of the light are the ones that have the longest wavelength. Following these two colours, we find yellow and green, with more energy than the previous ones. The colours that have the shortest wavelength are blue and purple.

The blue colour of the sea

When light finds itself with the ocean and seawater, water molecules absorb the longest wavelengths (the yellow, orange and red colours) the first. The next colour to be absorbed by water is green. So, the blue colour, which hasn’t been absorbed yet, is the one that is reflected and reaches our eyes.

The colours with the shortest wavelengths, which are blue and purple, can travel farther away through the water before being absorbed. Before this takes place, these colours are scattered by the water molecules, making the sea appear blue.

This phenomenon takes place at the sea due to the big quantity of water there is, but it doesn’t reproduce in situations of a small water accumulation. A clear example can be seen in a cup of water, where the water will always appear transparent.

Why is the sea darker blue in some places?

As we can observe, the sea hasn’t a unique hue of blue colour in all of its points. There are parts of the sea that appear in a darker colour. What is this phenomenon due to?

The sea has a darker colour in some zones because of its depth. As light travels through water, a moment arrives in which even blue light starts to be absorbed. As light blue has a longer wavelength than darker blue, light blue is absorbed first, leaving the darker hues of blue to travel deeper and making the ocean appear of a dark blue colour.

A moment arrives in which visible light has travelled so deep that all of it is absorbed, and the sea appears completely dark. This moment arrives when light has gone through about 1000 metres in the water. Below this depth, the sea appears black

Why is the ocean light blue in some places?

On the contrary of the marine depths, where the dark blue and even black predominate, in a big part of the coastal and shallow zones of the sea predominates a light blue or turquoise colour. This is due to that they are zones of little depth, in which there aren’t almost any particles suspended in the water, and, moreover, the marine floor is composed of white rocks and sand.

In these zones that are not too deep, there are fewer water molecules that can absorb light. When the blue light is reflected from the white depths of the sea, this colour is intensified in our eyes. In some zones, the water can be so shallow that the water molecules can’t even absorb green light, creating the green and turquoise hues that we can see in many places of the planet.

Other colours of the sea

The colour of the seawater can also be affected by what is suspended in the water. The seabed is full of all types of particles and life, which can change the colour in which we see the sea.

Sometimes the water is full of sediments, such as the sand that has been risen from the bottom of the sea after a storm. These sediments will reflect diverse colours of the light, making us perceive the water in a brownish colour.

The water of the sea can also turn green when there is phytoplankton. The latter is a micro-organism, at the origin of the food chain, that contains chlorophyll. Chlorophyll is the pigment that gives plants their characteristic green colour, absorbing all colours of the visible light spectrum except green light. So, when these micro-organisms are sufficiently near the marine surface, and green light hasn’t yet been absorbed by the water molecules, the phytoplankton absorbs all the blue colours of light, and the green light is reflected upwards, until reaching our eyes.


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