Why do breaking waves look white?

08 July 2014


A breaking wave on the ocean



Why do breaking waves look white?


This week, we've been dreaming of warm sandy beaches while trying to answer this question from Emma and Greig Robinson from New Zealand...

Emma - We're looking at the waves on Waikiki Beach in Hawaii at night. Wondering why the breaking waves are white. Where does the white come from? Thank you and we look forward to hearing your answer.

Georgia - So, where does the white in breaking waves come from? We asked bubble expert, John Nees, a physicist and optics researcher at the University of Michigan who cleared the matter up with a little bit of help from his engineering student daughter, Clara.

Clara - Like tiny ice crystals of snow, the tiny droplets and bubbles you see in the crest of a breaking wave appear right when they're grouped together even though they're transparent individually.

John - Light scatters in many directions as it passes from air through its single droplet of water. Individually, each droplet scatters the surrounding light just a little bit, but collectively, the droplets scatter a great deal.

Clara - This scattering mixes together all the different colours of light from the surroundings, blending them into white light.

Georgia - So, coloured light bounces in and around tiny bubbles in the water which mixes together to look white. So, why do we see this effect in breaking waves, but not in calm water?

John - It's important to note that light scatters when it passes between different materials such as water and air. The water in the ocean only meets the air at one surface, but the droplets in the surf meet millions of surfaces. So, they scatter the light more in a small area.

Clara - That's why only the part of the wave that has broken apart appears white.

John - Scattering makes the wave crest white with light from the surroundings. In some places though, there may also be light coming from inside the water. The crashing motion of the waves can agitate bioluminescent microorganisms. At night, you could see them glowing blue green in the surf.


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