Wednesday, July 18

Campus Queries: Why is the ocean blue if water itself has no color?


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Campus Queries is a new Q&A series in which Daily Bruin readers send in science-related questions for UCLA professors and experts to answer. This week, Daniele Bianchi, assistant professor of atmospheric and oceanic sciences, answers.

Q. Why is the sea sometimes blue or green when the water has actually no color? – Doris C.

A. If the ocean was pure water, it would still look blue, and that’s because the white light from the sun is made from all the different wavelengths. The water tends to absorb some of these (wavelengths), but not the blue. The ocean itself, and pure water, is bluish. It tends to let blue light pass through it more or scatter it more. (In) a glass of water, there’s not enough of water to scatter the light; you just need a lot of water to see the effect.

Light is this collection of different wavelengths, and when a photon interacts with a particle, different things can happen. Scattering is an optical interaction of light with the particle (and) is really (the) interaction of the radiation with small particles. With very small molecules, like molecules of water or air, blue light is preferentially scattered. (Scattering) depends on the size of radiation (waves).

There are other things in the water that absorb light, like phytoplankton. These are aquatic, unicellular plants that are (found) in exceptionally large numbers in the water. They have chlorophyll and other pigments, and the chlorophyll absorbs the blue and the red (wavelengths) very well, but less (of) the green. If there is a lot of phytoplankton, the water might look green. Phytoplankton are always trying to get the light, but water absorbs the light too, so there’s this interplay.

Some plankton can cause red tides, (like) specific phytoplankton that have pigments that scatter red light and absorb green and other wavelengths. It’s all a matter of (which) wavelengths are absorbed by the water and (which) wavelengths are absorbed by the plankton.

When there is a lot of runoff from water, like these days, there’s also sediments in the water. They absorb all sorts of frequencies and the water looks brown. So it really depends on what is in the water. Also, when you are sitting on the beach and looking at the water, there’s also reflection from the sky, which is why (the water) appears blue.

Have a science question? Submit your question to Campus Queries or email us at [email protected]

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