Several people have described the colour of the ocean just as it appears to them – James Joyce called it green, Lord Byron averred it was dark blue, and to Homer, it appeared wine red.
But is the colour of the seas subject to individual opinion? Or is there a scientific explanation for it? Consider the facts.
The ocean looks blue because red, orange and yellow (long wavelength light, and the first three of the colour spectrum ROYGBIV) are absorbed more strongly by water than blue (short wavelength light). So when white light from the sun enters the ocean, it is mostly the blue that gets returned. Why?
Pure water is clear. However if it is deep enough, so that light cannot reflect off the sea floor, it appears dark blue. This is largely because of some basic physics.
Water molecules are better at absorbing light that arrives in longer wavelengths, meaning the reds, oranges, yellows and greens. This mostly leaves the blues, which have shorter wavelengths. As blue light is less likely to be absorbed, it can penetrate to deeper depths, making deep water look bluer.
Also, light at a short wavelength is also more likely to be scattered or deflected in different directions, including back out of the water towards our eyes, making the sea appear blue. Further, the cones of the retina react more strongly to light of red, blue and green wavelengths than others. And that’s the physics of it.
But even more important is the biology, because the biggest impact on sea colour is made by tiny organisms called phytoplankton. Phytoplankton absorb electromagnetic radiation in the red and blue parts of the visible light spectrum, but reflect greens, which explains why seas in which they are thriving appear greener.
It happens that the blue light is absorbed more, while the yellow pigments from these plants mix with the blue light waves to produce the colour green.
Sometimes though, parts of the oceans will look milky brown after a storm passes. This is because winds and currents associated with the storm churn up sand and sediment from the rivers that lead into the oceans.
See Also: So Tell Me, Why is the Sky Blue?
