Cannell, David S. Department of Physics, University of California, Santa Barbara, California.
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An optical method of rendering fluid flow patterns visible by using index-of-refraction differences in the flow. As a simple example, sunlight passing through the air above a warm toaster may be seen to form darker and brighter streaks on a surface a meter or two from the toaster. The method relies on the fact that rays of light bend toward regions of higher refractive index while passing through a transparent material. The fluid is usually illuminated by a parallel beam of light. Figure 1 depicts the method as it might be applied to a fluid sample undergoing thermal convection between two parallel plates, with the lower plate being kept warmer than the upper one. As illustrated, the rays bend toward the cooler down-flowing regions, where the refractive index is higher, and away from the warmer up-flowing ones. After they have passed through the fluid layer, the rays tend to focus above the cooler regions and defocus above the warmer regions. If an image of the light beam is recorded not too far from the sample, brighter areas of the image will lie above regions of down flow, where the rays have been concentrated, and darker areas will lie above regions of up flow. If an image is recorded at a distance greater than the distance for which the rays actually focus, then it is possible to have a situation in which darker, rather than brighter, areas lie above down-flowing regions. In practice this is not a problem, because the image can be observed beginning near the sample and gradually moving away, until a suitable situation is achieved. Because the light passes completely through the sample, the bending effect for each ray is averaged over the sample thickness. See also: Convection (heat); Refraction of waves
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