Why does the ocean appear blue?

-Karthik Gurumurthy

A lot of people think the ocean looks blue because it’s just reflecting the sky, but that’s not really the main reason. The real reason is how water handles light. Water tends to absorb the colors with longer wavelengths — like red, orange, and yellow — much more than it absorbs blue, which has a shorter wavelength. So when sunlight hits the ocean, most of the warmer colors get absorbed, and the blue light gets scattered back, which is what we end up seeing. Funny enough, that’s actually pretty similar to why the sky looks blue too.

Even though the ocean and the sky both look blue, they don’t depend on each other for their color. In both cases, it’s all about how longer-wavelength light gets absorbed, leaving the blue behind. One thing though — this effect really shows up only when the water is clean. If the water’s full of mud, algae, or a bunch of other stuff, all that extra junk scatters the light differently and can make the water look murky instead of blue.