HOW COME?: The ocean is blue

Why is the ocean blue? asks Trisha Joshi, a student in Brookville, NY.

Water is a liquid chameleon. A glass of purified bottled water? Clear and colorless. A pond on an overcast fall day? A moody blue-gray. And the water off a sunny Caribbean beach? Tropical turquoise. Some think it's the sky that gives oceans, lakes and ponds their shifting blue color. But as it turns out, a glass of plain, pure water actually has a definite color -- even if we can't perceive it.

How come? Look at the air around you; you can see through it, and it appears colorless. Now look up into the sky on a sunny day, at the mass of air above you. It's the same air. But now it looks blue.

As it turns out, pure water is also blue. However, since there is so little water in a drinking glass, its intrinsic color is just too faint to see. But fill a clear glass building with the very same water, and the blue tint would be obvious.

Luckily, we can see water's true hue in a more compact container. Researchers at New Hampshire's Dartmouth College filled a slender tube with purified water, capping the end with clear plastic. The tube was nearly 10 feet long and about 1.6 inches across. The tube's length meant that light passing through should encounter enough water molecules to lose a significant amount of its red -- enough to make the color of water easily visible.

And that's exactly what happened. Observers reported seeing blue. And a camera captured the pale, robin's egg blue for the rest of us.

The blue of water depends on how its individual molecules absorb light. White light, such as sunlight, is made of a hidden rainbow of colors: red, orange, yellow, green, blue, indigo, violet. Earth's sky is blue because blue wavelengths of light are scattered out of the white light beam by the electrons in gas molecules.

Scientists say the way light and water molecules interact may be unique. Unlike the sky, water is "vibrationally colored." Water molecules selectively absorb much of the red end of the light spectrum. When sunlight passes into the ocean, reds vanish into water molecules, leaving blues to travel on.

But we don't have to look into the depths of the ocean to see blue. Tiny particles floating in the water helpfully scatter the blue color out to the eyes of those looking at the water's surface. Sky reflections make water appear even bluer -- or, on an overcast day, steel-gray. Ocean colors also change with the amount of living plant material suspended in the water. Green chlorophyll in the tiny, single-celled plants known as phytoplankton create vast swathes of green and blue-green across the Earth's seas, especially near coastlines.