The Debunker: Does the Ocean Look Blue Because It Reflects the Sky?

by Ken Jennings

June is the time of year the United Nations observes World Oceans Day and the U.S. celebrates National Oceans Month, so we’ve asked Skipper Ken Jennings to navigate us through four maritime myths that refuse to die. It turns out that none of them really hold water.

Ocean Myth #2: The Ocean Is Blue Because It Reflects the Sky.

I remember noticing as a child that a glass of water from the kitchen tap was colorless. So why were lakes and oceans blue? My parents told me that the blue color was due to the surface of the water reflecting the sky, and I believed them. Sure enough, on cloudy days (which were plentiful in Seattle, where I grew up) the lakes looked more gray than blue.

But my parents lied to me, friends—and maybe yours did too. Look at a photo of the Arctic or Antarctica: even on the grayest of days, glacial ice can have the kind of brilliant cyan blue you rarely see outside of an NBA jersey from the 1990s. The fact is that water is not colorless.

Due to the harmonic vibration of the oxygen-hydrogen bonds in water molecules, it happens to absorb long wavelengths of light (red, orange, yellow) much better than it does shorter wavelengths, like blue light. This tendency toward a light greenish-blue color is weak enough that you don’t see it in a glass or a bathtub full of water, but it becomes noticeable as light passes through thicker samples, as in a lake or ocean.

Of course, water’s inherent color doesn’t account for all of the blueness of the ocean. Reflection from a blue sky can deepen the color of water, especially from certain viewing angles. So can mineral impurities dissolved in the water. In fact, if not for impurities, bodies of water wouldn’t be blue at all — light hardly penetrates the ocean at all below about 650 feet, so all deep bodies of water would be completely colorless if not for light being scattered around by particulate matter near the surface. “In 1492, Columbus sailed the ocean… black?” Not only does it not sound picturesque, it doesn’t rhyme at all.

Quick Quiz: On what Italian island in the Gulf of Naples would you find the famously colorful sea cavern called the Blue Grotto?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.