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June 9, 2022

The Origin of Ocean Blue

The physics behind why we can call Earth the “Blue Planet”

Our little Earth shimmers fragile, bluish in the infinite black. The view from space quickly proves the common label “Blue Planet”. Two thirds of the Earth´s surface is covered by oceans. But why do they appear blue when water is transparent? And doesn’t it actually have something to do with the blue of the sky?

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Red lips, yellow vests, or blue ink – most of the colors we see every day are determined by reflection, scattering, and absorption of light. At first glance, the principle seems counterintuitive: We see blue ink as blue precisely because the ink absorbs other colors such as yellow, green, and red and only reflects the blue light. It works similarly with the sea. Blue light is absorbed the least by water. As a result, it can penetrate deeper waters, to a depth of about 100 meters. Light is generally scattered by sediments and microorganisms in the water. With increasing depth this is predominantly blue light. That is why we see waters blue and deeper waters bluer.

“You often hear that the blue ocean reflects merely the blue sky. But that´s not true. Both are independently blue. For both, the explanation has to do with light scattering, which describes how the path of light rays is deviated into different directions when encountering particles,” Nathalie Agudelo points out a common misconception. The PhD student is working in the Danzl research group at the Institute of Science and Technology Austria (ISTA) and studies novel microscopes to better observe biological processes. “The molecules of the atmosphere scatter blue light more strongly than red light, basically because blue light has a higher frequency. So, if we look at a random spot in the sky, we see the scattered blue light reaching us from there. Also, we have all been amazed by colorful sunsets when the sun is approaching the horizon. In this case, the sunlight has to travel a long distance through the atmosphere, and therefore, the blue parts are scattered away and it’s mostly the orange-red unscattered light that we see.”


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