Why Is The Sea Blue? (Blue Ocean Explanation)

Blue Sea Explanation

Have you ever wondered why you’ve looked at an enormous expanse of water, a lake, or an ocean that appears very blue? Sometimes beautifully blue, like in Cuba or Jamaica or somewhere where the beaches are lovely white sand, and the water is a beautiful blue. But then, if you take that water and cup it in your hands, it’s not blue. It’s crystal clear! If you ever wondered why that is, the answer lies in water’s chemical & physical structure.

Why Is The Sea Blue?

The ocean is blue because light interacts with the water as light travels from the sun through the water. It either gets absorbed by particles or is scattered. As the light travels, it gets attenuated, which is a fancy word for as you go deeper into the water, more and more light is lost to absorption. The reason that absorption affects the color of the water is that different wavelengths of light are absorbed in various rates.

So red light, which is longer and has slower wavelengths, is more likely to hit a particle. Therefore is attenuated faster than blue and green light. Also, blue and green light travels deeper into the water, giving it its blue color. This light is absorbed as it travels through the water, explaining why tropical waters are a very different color, blue, than temperate waters.

It is because tropical waters are very low productivity, whereas temperate waters have very high productivity. It means there is much dissolved organic matter in the water and plankton. These two things give the light many more particles to hit and be absorbed. It means that the light gets absorbed and attenuated much more quickly in temperate waters than in tropical waters.

Therefore tropical waters are a lot bluer because there is more light. Tropical waters are also a lot shallower, so the light travels to the bottom. It also reflects the white-colored sand. So thus, the light traveling through the water not only explains why the ocean is blue. But why do different parts of the sea appear to be different colors of blue?

Blue sea explanation by physics

The shorter wavelengths of light on the blue end of the spectrum can pass through. But the longer wavelengths of light on the red end of the spectrum can not pass through. When white light containing all the rainbow colors passes through, only the blue wavelengths come out the other side, and the gel looks blue.

Blue wavelengths travel much better through water than red wavelengths. Light travels into the water and down to the bottom of the sea. The light that reaches the bottom has a much less red wavelength than the light at the surface. The light reflects off the bottom and travels back to the surface to reach our eyes. There is even less red. So the water looks even bluer.

If you look at the ocean, the water is deeper, so the bottom is further down. The light travels further before reflecting off the bottom and coming back up, so the blue is more prominent. In bottomless water, it appears quite a deep blue. In effect, the water is blue, but it takes a lot of water to see the color.

Why does some water look green? If the ocean has many planktons, the green hue caused by the plankton is much stronger than the soft blue color. So the blue is overpowered by the green. The blue color of ocean water makes underwater filming tricky because the light down there doesn’t have a lot of red. Even things brightly colored with reds or yellows don’t look that way to our eyes or camera.

Blue ocean explanation by chemistry

A large body of water appears blue because it absorbs red light due to its chemical structure. When a photon with the amount of energy that makes it red hits a water molecule, it’ll absorb and allow the water molecule to vibrate with its OH bonds at a more excited state. The result is that you see water looking blue.

Why don’t you see that it doesn’t look blue when you cup your hands and hold water in your hands? It’s because it’s not enough water. The effect is not visible in small water quantities, like the amount of water you hold in your hands. Here’s a neat fact if you took a gallon of water and stretched it out into a long thin tube and looked through the tube at one end, the water would look blue because you’re looking through enough water to get that effect.

Some other factors can influence the effect.

  • The reflection of the blue color from the sky on the water.
  • The scattering of blue light from particles.

It might be suspended in the water like runoff from glaciers or things like that. In general, when you see a large body of water that appears very blue, it’s because the water’s chemical structure has caused it to absorb the red light, leaving the water looking blue. If you have any other questions about the ocean, leave them in the comments below.

Light Scattering

Learn More:

Why Is The Sky Blue?

Why Is The Sea Water Salty?

Why The Red Sea Is Red?


“What’s the difference between an ocean and a sea?”. Ocean facts. National Oceanic and Atmospheric Administration.
Stow, Dorrik. Encyclopedia of the Oceans. Oxford University Press.
Nishri, A.; Stiller, M; Rimmer, A.; Geifman. “Lake Kinneret (The Sea of Galilee): the effects of diversion of external salinity sources and the probable chemical composition of the internal salinity sources.” Chemical Geology.
Conforti, B.; Bravo, Luigi Ferrari. The Italian Yearbook of International Law, Volume 14. Martinus Nijhoff Publishers.
Karleskint, George; Turner, Richard. Introduction to Marine Biology. Cengage Learning.
American Society of Civil Engineers (eds.). The Glossary of the Mapping Sciences. ASCE Publications.

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher. I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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