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Why Is The Ocean Blue? Here Are The Facts

There are so very many things to wonder about the ocean. It’s almost as big a mystery as outer space, even though it’s so much closer and more accessible to us. Luckily, we do know the answers to some of the mysteries surrounding the ocean. For example, why is the ocean is blue…?

Red, orange, yellow, and green light have long wavelengths and less energy, and are absorbed when they hit the water’s surface. Blue, indigo, and violet light have shorter wavelengths and more energy and can go beyond the water’s surface, becoming scattered and remaining visible to our eyes. This is why we see the ocean as blue.

If you were to scoop water from the ocean into your hands or into a glass, you would see that it is not actually blue. Ocean water is colorless, just like drinking water, unless it is dirtied with sediment and other solid materials. So, the real question is why does the ocean appear to be blue when looked at as a whole body of water.

Why Does The Ocean Appear To Be Blue?

The blue appearance of the ocean is the result of light from the sun interacting with the ocean water. The National Oceanic And Atmospheric Administration says that “The ocean acts like a sunlight filter”. To understand this properly, you need to know about the visible light spectrum.

The Visible Light Spectrum

There is a whole spectrum of electromagnetic radiation reaching the earth from the sun. It includes gamma rays, infrared radiation, x-rays, microwaves, etc. However, our human eyes can only see a small portion of this spectrum—the portion known as the visible light spectrum.

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Light is measured according to its wavelength. Wavelength is the distance from one wave peak to the next or one wave trough to the next. Visible light has a wavelength range of between 380 nanometers and 700 nanometers.

Visible light is perceived as white, but it is actually made up of seven different colors. These colors are absorbed, refracted, and reflected differently by various objects or mediums (like water), which is how we see these things as different colors. 

On the electromagnetic spectrum, the different types of radiation are listed in order of decreasing wavelength, so you would also list the colors of the visible light spectrum in order of decreasing wavelength. This order is red, orange, yellow, green, blue, indigo, and violet. 

The Relationship Between Light Wavelength, Frequency, And Energy

Wavelength is related to frequency, and frequency is related to energy. The shorter the wavelength, the higher its frequency. Higher frequencies equate to more energy. This means that light with shorter wavelengths (and, thus, a higher frequency) has more energy.

How Does The Relationship Between Light Wavelength, Frequency, And Energy Make The Ocean Appear Blue? 

Red, orange, yellow, and green light, with their longer wavelengths and lower frequencies, have smaller amounts of energy. Thus, they are absorbed by the water almost immediately when they hit the water’s surface.

Light with more energy (shorter wavelengths), like blue, indigo, and violet of the visible light spectrum, is able to penetrate the surface of the water and travel further, becoming scattered and remaining visible to our eyes. Thus, we perceive the color of the ocean as being blue.

Why Is The Ocean Darker Blue In Some Places?

The ocean appears darker in some areas depending on the depth of the water

Now, the ocean is not just one uniform blue color. There are parts of the ocean that are darker blue. Why is this?

The ocean is darker blue in some places because of the depth of the water in those spots. As light travels further and further into the ocean’s depths, more and more light is absorbed

Even the blues start to be absorbed, and the lighter blues, with their longer wavelengths, will be absorbed first, leaving the darker blue light to continue further and making the oceans appear darker blue.  

Eventually, as it travels deeper and deeper, all visible light will be absorbed, and the ocean water appears black. 

Why Is The Ocean Lighter Blue In Some Places?

You’ve seen pictures of beautiful Mediterranean coastlines with their turquoise oceans. What makes these oceans lighter blue? Does the depth of the water in those locations play a role?

Depth does play a contributing role in the areas where the ocean appears light blue. But it also has to do with the fact that, in these places, there is often very little matter suspended in the water, and the floor of the ocean is white rock or white sand. 

Light hits the water, and the red waves are absorbed while the blue waves penetrate and are scattered, just like normal. However, the light color is intensified to our eyes as the light waves are reflected back up again by the white ocean bed. 

What Else Affects The Color Of The Ocean?

The color of the ocean is also affected by what is suspended in the water. When light rays strike matter, they are absorbed, scattered, and reflected. We have already spoken about how this happens when light hits the water particles, but the ocean is full of all kinds of other matter. 

When there is a lot of sediment in the water, particularly near the surface of the water, then the red, orange, and yellow light waves can be reflected off of these particles before being absorbed, and you see the ocean as a dirty brown color. 

In the ocean, there are microorganisms called phytoplankton. Even though they are so tiny, phytoplankton can affect the perceived color of the ocean water. But how?

Phytoplankton are photosynthesizing creatures that contain chlorophyll. Chlorophyll is a pigment that absorbs all light waves except green ones. It is the same pigment that makes plants green. 

So, when these microorganisms are close enough to the surface of the water that green light has not yet been absorbed (green light travels further than red light), then blues are absorbed by the phytoplankton themselves, and green light is reflected back up.

Green Oceans May Be The Key To Climate Change

Scientists are actually very interested in the green parts of the ocean, or more specifically, they are interested in the phytoplankton in these areas. 

Because phytoplankton photosynthesize, it means that they use up carbon dioxide from the atmosphere and release oxygen. Less carbon dioxide in the air counteracts the greenhouse effect and lowers the temperate of the atmosphere.

So, by keeping an eye on the phytoplankton in the green parts of the ocean, scientists can predict environmental changes. 

What Colors Can You See From Under The Ocean’s Surface?

Some areas of the sea appear green

According to Exploring Our Fluid Earth, deep-sea divers perceive most of the creatures and formations as various shades of blue or black, but when a white light is shone directly on them, they become all kinds of different colors. 

This is because only blue light from the sun is reaching these things naturally, but when you shine a white light, the full visible spectrum reaches them, and their true colors are reflected back to our eyes. 

Some deep-sea creatures are adapted to use the way light is absorbed and scattered by the ocean water to their advantage. Many of these creatures are shades of red and brown. However, at the depth at which they live, no red light waves are able to reach them. So, instead of appearing bright red, they look black and are thus able to hide better from their prey or predators. 


The ocean water is not blue, but the ocean as a whole body of water appears to be blue because low energy red light waves are absorbed by the ocean. In contrast, high-energy blue light waves penetrate and are scattered by it, remaining visible to our eyes.

The deeper the ocean, the more light is absorbed as it travels further down, and the ocean water will appear deep blue. In some areas, like the Mediterranean, the white rocks and sand at the bottom of the ocean reflect the clue light back to our eyes, making the water appear brilliant shades of light blue and turquoise. 

The presence of sediment in the ocean water can cause a brown hue to the ocean, and the presence of phytoplankton can make the ocean appear to be green. 

How amazing is this planet that we get to call our home?

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