Light travels at different speeds through different media. Water slows light down more than air, for instance. Corn syrup slows it down even more than water. In this experiment, we create a gradient solution of sugar water in a tank. As a laser beam travels through the liquid, the changing concentration bends its path!
The solution at the bottom of the tank is about 80 percent sugar by volume. As a siphon slowly transfers liquid into the aquarium, we add more and more water to the top container, diluting the mixture. The last layers to be added contain almost no sugar.
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Sugar water is denser than pure water. This property keeps the heavier, more concentrated solution on the bottom of the tank. Lighter mixtures float above in order of density. We chose a siphon system to stack our liquids because it transfers the fluid slowly and gently, without creating too much turbulence. This method allows a relatively smooth density gradient to form.
Our variable solution of sugar water also has a gradient index of refraction, meaning that light travels faster in some parts than others. Namely, light is slowed down the most in the heavy 80% region towards the bottom. It travels faster as the mixture becomes more dilute near the top of the volume of fluid.
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Imagine a line of children holding hands and running across a field. If they all run at the same speed, the line stays straight. What happens if they’re not all identical athletes, and you place them in order of running speed? The line bends. One end of the chain ends up ahead of the other. This is a bit like what we’ve done to the laser beam.
Written By: Caela Barry

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