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Visualizing Ultrasound with Schlieren Optics Part III

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 Physics   |   Science
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Part 3 of 3. We combine the stroboscopic effect with the use of color filters in our schlieren optics setup to visualize the changes in air pressure that occur in a standing wave created by an ultrasonic transducer (driven at 28 kHz) and a reflecting glass plate. As a finale, we levitate small Styrofoam balls in the pressure antinodes.

Without the color filters, the brightness of the schlieren image only gives us information about the magnitude of the pressure changes in the standing wave. We can now use the color information to tell us whether the pressure is increasing or decreasing. Each colored band is a pressure antinode that is oscillating between high and low pressure.

The order of the color bands in the standing wave depends on the relative phases of the strobe light and the transducer. Although we do not show it here, shifting the phase of the strobing light source by 180 degrees causes the colors in the standing wave to reverse.

The color filters we use here are Kodak Wratten 2 color filter #29 (red) and #61 (green).

More information about our schlieren optics set-up and how it works can be found on our Science Demonstration website: https://sciencedemonstrations.fas.harvard.edu/presentations/schlieren-optics

Safety Note: Although 28 kHz is beyond the range of human hearing, ear protection should be worn whenever attempting this experiment to avoid damaging vibrations in parts of the ear. Any high-pitched whines you hear in this video are not dangerous.

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