Why is the sea blue?

by B. N. Sullivan

The name of this blog is The Right Blue, and as we explain on our About page, that term refers to a quest:

Sea water viewed from beneath the surface comes in many hues and shades. Surfers wait for the perfect wave; divers seek the right blue.

By the way, the color of the water in the photo on this page is just about as close to 'the right blue' as we've ever seen. The picture was shot at a dive site called Bloody Bay Wall, on the northern shore of Little Cayman, Cayman Islands. The image was scanned from a transparency (slide), but the color has not been tinkered with at all.

So why does the sea look blue? And why does it sometimes look not-so-blue, or even green?

The sea looks blue for the same reason the sky looks blue: the refraction and scattering of light rays. Light scatters as it passes through air or water. Since blue light has a short wavelength, it scatters most easily. Conversely, red light has a long wavelength and scatters less easily. So, the blue rays get reflected back, and that's what we 'see' as the color of the sky or the water surface.

Notice I said water surface. So far I'm talking about what color a body of water appears to be when viewed from the shoreline, or from the deck of a boat.

Water viewed from beneath the surface appears in different shades depending on the depth, and on how much particulate matter is suspended in it. If you descend just a few meters in clear water on a sunny day and then look up toward the surface, the water will look almost transparent with perhaps a tinge of blue. Descend a few more meters and look up toward the surface again, and the apparent color will seem more blue. Descend below, say 25 meters, and you will see something approaching the color of the water in the photo above.

If the water has a lot of 'stuff' in it -- stirred up sand, or plankton, or tiny bits of debris -- some of the light will be deflected. The water will look less transparent -- possibly even murky. Likewise, if the sky above is overcast, even clear water will sometimes seem grayish from below.

As a rule of thumb, the deeper you dive, the darker the water appears, because fewer light rays are able to penetrate the depths. On night dives, there is no ambient light under the surface, so everything is inky black. Divers use artificial light from a flashlight or spotlight to illuminate a small area at depth, or during a night dive.