Why is the ocean blue?

Last week’s post explored why the sky above is a beautiful blue. In short, it’s because blue light gets scattered more in the atmosphere compared to the other colours of light. But then, why is the ocean, or other water, blue?

It’s not just because it reflects the colour of the sky (although this does have some effect). If so, then no indoor water would appear a shade of blue. Instead, it’s actually a very similar reason to why the sky is blue.


When white light comes down from the sun, some of the blue is scattered in the atmosphere, but some still gets down to us here with the rest of the light. When this light hits the water, the longer wavelengths of light with lower energy, the reds and oranges are absorbed by the water molecules. The blue isn’t, and it is reflected back up through the surface.

Interestingly, the absorption of reds, oranges, yellows, etc… is due to the vibrational energy patterns of the standard H2O molecule. Heavy water, D2O, containing neutrons within the nucleus of the hydrogen atoms, has different vibrational energy patterns and does is actually entirely colourless.

And if you remember scattering from last week, the blue light is also scattered throughout the water more than the other colours of light and this also contributes to the blue appearance of the water.


Other contaminants, such as chlorine in pools, salt in the ocean, or mud and algae in ponds and lakes naturally have an impact on the colour of the water and can easily overwhelm or modify the natural blue shade of water.

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