Earth is famously known as the blue planet due to its abundant water, but why is the ocean blue?
The ocean is blue to the human eye because it absorbs and scatters light. The water acts as a filter, taking in colors from the red spectrum and mirroring back colors in the blue one.
However, phytoplankton also plays a prominent role in how blue or green the ocean appears.
In today’s article, we’ll dive into “Why is the ocean blue?” by looking at light, depth, and phytoplankton.
Why Is The Ocean Blue?
“Why is the ocean blue,” is a fair question. After all, water is clear.
The reason for this phenomenon is light absorption. Let’s look into it.
How Does Light Absorption Make The Ocean Blue?
Unlike water, light is not clear but full of color. However, you don’t notice light’s spectacular hues very often unless there is a rainbow.
To make a rainbow, light passes through water droplets and is scattered, appearing as colors to our eyes. Each color has a frequency range in the light spectrum that is measured in nanometers (nm):
| The Visible Light Spectrum | |
| Color | Wavelength |
| Red | ~ 625 – 700 nm |
| Orange | ~ 590 – 625 nm |
| Yellow | ~ 565 – 590 nm |
| Green | ~ 520 – 565 nm |
| Blue | ~ 500 – 520 nm |
| Indigo | ~ 435 – 500 nm |
| Violet | ~ 380 – 435 nm |
The complete electromagnetic spectrum encompasses all types of light, including radio waves, microwaves, infrared, visible light, ultraviolet light, X-rays, and gamma rays. Hint: color is visible light!
Yet, despite water droplets gifting the world with gorgeous rainbows and the sea being made up of water droplets, the ocean does not appear as a giant map to find a leprechaun’s treasure.
But that’s because droplets don’t only scatter light; they absorb it, too.
How Does Light Absorption Work?
Color is created by white light absorption. No light absorption? White. Total absorption? Black. But most light absorption falls in between these extremes, giving us color. It all depends on what is reflected and what is absorbed.
Water droplets don’t absorb light very efficiently. It takes a significant amount to absorb enough light to create a color our eyes can see.
The easiest colors to absorb are the “low energy” ones, such as reds, oranges, and yellows, which have longer wavelengths than violet, indigo, and blue.
Blue has high energy and a short wavelength. Thus, the answer to “Why is the ocean blue?” is that it takes much more water to absorb blue wavelengths than red and yellow colors.
Why Are Longer Light Wavelengths Easier To Absorb?
The lower the energy, the easier a wavelength is to absorb. Light is a lot the tortious and the hare.
Like a tortious, reds and yellows have slow and steady energy and create a longer wavelength light. Blues are like a hare, fast and short. Yes, the tortious won the race, but it would be easier to catch.
In less than 4 inches of water, infrared light is “captured.” By 65 feet, the red light is essentially gone.
Thus, any red fish are nearly impossible to see in depths below this, helping them evade predators. However, blue light is still penetrating at 656 feet.
How Does Depth Impact The Ocean Color?
The depth of the water impacts the shade of blue. The deeper the water, the more color is absorbed, creating a denser and “darker” blue color.
In addition, in significantly deeper water, there is no reflection of light from the sea floor, which makes the color blue look navy.
In fact, most of the ocean is entirely dark. Hardly any light penetrates deeper than 656 feet, and no light penetrates deeper than 3,280 feet.
That’s why shallow water is more likely to look clear or have a turquoise color as light passes through and bounces back off the floor. Hence, blue ocean water looks clear when kept in a glass of water: light is coming in from all angles.
How Does Algae (Phytoplankton) Impact The Ocean Color?
Cells of phytoplankton, which are single-celled algae and cyanobacteria, reflect green light thanks to containing chlorophyll. They impact the ocean’s color, giving it aqua and green pigments, sometimes even some shades of brown.
How much influence the phytoplankton has on the ocean’s color depends on the following:
- Type of phytoplankton population, as they vary in shade of green
- The amount and concentration levels of phytoplankton
Other impurities can also impact the ocean’s color, such as marine life, plant material, and sand churned up. That’s why smaller bodies of water will have a different color.
Is The Ocean Blue Because Of The Sky?
The color of the ocean doesn’t look blue because it reflects the blue sky. So, why is the sea blue then?
When light passes through our planet’s atmosphere, it has to pass through gasses and dust particles. Long wavelengths of light, such as red, easily pass through these gasses. However, the shorter wavelengths, like blue and violet, are disrupted, hence the term scattering of light.
That’s basically why the sky is blue. Or red, depending on the time of the day.
The color of the sea comes from light absorption and scattering—water molecules absorb all other wavelengths but the blue and violet, scattering them around.
However, less light reaches the Earth when the sky is gray, making the ocean look steel blue.
What color is the ocean then? Well, the water color is clear, so in theory, while the ocean may appear blue, it is actually clear.
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