Why Is the Ocean Blue?

Ocean waves lap the sand on the beach. — (Image credit: Galyna Andrushko | Shutterstock)

It's commonly believed that the ocean is blue because it's reflecting the blue sky. But this is a misconception.

The ocean is blue because of the way it absorbs sunlight, according to the National Oceanic and Atmospheric Administration (NOAA).

However, NOAA notes that the ocean may take on other hues, including red and green, if light bounces off objects floating near the surface of the water, such as sediment and algae.

Just how blue the water is depends on how much of it is available to absorb the light.

For instance, water in a glass is clear — there aren't enough water molecules to really absorb the light.

But ocean water appears bluer the farther you travel down the water column. The water molecules absorb infrared, red and ultraviolet light first, and then yellow, green and violet.

Blue light is absorbed the least, giving it the greatest ocean penetration depth, according to NASA.

This fact is clear if you look at unedited underwater photos that weren't taken with a camera flash or another artificial light source — even the most vibrant of tropical fish look blue.

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Joseph Bennington-Castro is a Hawaii-based contributing writer for Live Science and Space.com. He holds a master's degree in science journalism from New York University, and a bachelor's degree in physics from the University of Hawaii. His work covers all areas of science, from the quirky mating behaviors of different animals, to the drug and alcohol habits of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had a near-obsession with video games for as long as he can remember, and is probably playing a game at this very moment.