Is the Ocean a Mirror? or Why is the Ocean Blue?

Did you actually make it through that?? Did you catch the strange reason why the sea is blue? Apparently the ocean is a mirror (not entirely false) that reflects the blue sky, hence it is blue (not entirely true). Some hypothesize that the ocean is blue because it reflects the blue sky, but this would only be visible at relatively low angles of observation and on flat water.

So why is the ocean blue? Water itself isn’t blue, right?

The most widely-held hypothesis is that blue wavelengths of light penetrates deeper while red wavelengths are rapidly absorbed by the water molecules and solutes. However, if you grab a cup and sample the ocean’s water, it will likely be clear. But pure water does have an intrinsic blueness that is all its own, caused by the vibrations of the water molecule. The “good” vibrations of water absorb light wavelengths in the red spectrum. Blue is not absorbed but instead scattered by the molecules and solutes in water. That is why we are able to see it!

How do we know it is vibration that gives water its color? The graph below (from WebExhibits) can aid us in demonstrating this phenomenon.


The top line is standard H2O, while the gray line below is a form of H2O known as “heavy” water. Both types of water are the same in all respects except for mass. Heavy water, which contains 2 deuteriums (hydrogen atoms with an extra neutron) attached to the oxygen, has a mass about 10% more than that of regular water and is colorless. Comparing the 2 curves on a graph of wavelength versus absorption, we can see that heavy water’s absorptive property is shifted into slower, longer wavelengths (out of the visual spectra) relative to the regular water. The only difference between the two types of water is a 10% change in mass which corresponds to shorter bond lengths between atoms. Greater mass means slower vibrations and less energy. With all other factors held constant we can observe that the pale blue color of standard water is due to greater vibration transition energy. Since heavy water is colorless, there is no absorption of visual wavelength, letting all light pass through.

While water at the surface may act as a mirror of the sky in certain situations, pure water itself is blue by nature. With deep water, the blue intensifies as it is scattered over more volume. This is why small rivers and lakes aren’t as blue as the ocean. The origin of the color of water is unique in that it is a phenomenon of vibrational transitions rather than an interaction of visible light with a substance’s electrons. Of course other colors can be caused by algal blooms and suspended sediment, or green dye in the case of Chicago’s Saint Patrick’s Day festivities…

Interested in learning more? See this online article from Braun & Smirnov at Dartmouth College.

A more accurate song…

Kevin Zelnio (870 Posts)

10 Replies to “Is the Ocean a Mirror? or Why is the Ocean Blue?”

  1. Corey is right, hydrogen (H or 1H) is defined by its single proton; deuterium (D or 2H) also carries a neutron. Tritium (T or 3H) carries two neutrons.

  2. I stand corrected. I was 2am when I finished this post and was working from memory! Thanks for keeping me honest.

  3. Whoever made that film has no doubt seen pictures of a tropical lagoon under a brilliant blue, with the water being emerald green. Apparently seeing one blue sky with blue water was enough for them to prove the assertion. Some people don’t learn from experience.

  4. In addition to more mass, heavy water has different bond lengths. (The extra neutron changes the relative mass of the nucleus)

    Fun fact: heavy water is toxic (in large quantities), because it doesn’t metabolize properly.

  5. Great topic! Why not call the article after the issue of why the ocean’s (sometimes) blue, though, Kevin? You’d probably get a bijillion hits, and we’d all be riiich enough to move to Dubai!

  6. Of course there is, as I now understand, wrong and wronger.

    About five years ago I was emailed info from a bloke who ran a website dedicated to the promotion of bottled water. To muffle the advertorial tone of the site he included various fascinating facts about water, the most mind-blowing of which was the fact that the sky is blue because it reflects the blue colour of the ocean below.

    That’s right – the sky is blue because it is reflecting the colour of the sea.

    Still makes me want to bang my head against the wall! In the end I just sent a message to BadScience and wept sky-coloured tears.

  7. “Why not call the article after the issue of why the ocean’s (sometimes) blue, though, Kevin? You’d probably get a bijillion hits, and we’d all be riiich enough to move to Dubai!”

    *blinks* Peter, your comment has gone over my head man.

    Paul, ROTMFFLMMFAO except I’m sure there 10% of the world’s population that would find that a reasonable argument.

  8. sorry, Kev. I meant to say that if you titled the story “why is the ocean blue” it would get a lot of traffic through search engines.

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