White Water Ask Dr. Knowledge Boston Globe

Ask Dr. Knowledge:
What Makes White Water White?

August 16, 2010
First let’s think about what you see when you look at clear, still water in a shallow pond. You see light that reflects from the bottom of the pond, and light that reflects from the surface. Every time there’s a change in the material the light passes through — air to water or water to bottom — you get a reflection.
If there’s an air bubble in the pond, you can see it due to reflections when a ray of light goes from water to air or air to water.
If the water flows slowly, there’s not much change in what you see. Ripples on the surface will reflect light in slightly different directions, but it won’t be a big effect.
Now suppose the water starts to move quickly. If the speed is fast enough, there is a transition between smooth so-called laminar flow and an irregular frothy state called turbulent flow. In this case, lots of air bubbles get mixed in with the water, and each acts as a tiny reflector, reflecting light in pretty much random directions that change with time. All the colors are reflected back pretty much equally well, but instead of a simple mirrorlike look, you just see white.
You can easily see this effect at home with a water faucet. A slow stream looks like a piece of glass, but if you run more water, eventually it breaks into a frothy turbulent flow.
How exactly does turbulence start? This is not well understood. We think we know the right equations, but they’re hard to solve; there’s a million-dollar prize offered essentially for just showing that they make sense. For prize information, check out www.claymath.org/millennium/NavierStokes_Equations/
Ask Dr. Knowledge is written by Northeastern University physicist John Swain.
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