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As the waves encounter obstacles in their path, their behavior can be observed by watching the movement of the dark and bright spots on the sheet of paper. As the water waves move through the ripple tank, the dark and bright spots move as well. So the bright spots represent wave troughs and the dark spots represent wave crests. A crest of water will absorb more light than a trough. A portion of light is absorbed by the water as it passes through the tank. A light typically shines upon the water from above and illuminates a white sheet of paper placed directly below the tank. A ripple tank is a large glass-bottomed tank of water that is used to study the behavior of water waves. The study of waves in two dimensions is often done using a ripple tank. But what if the wave is traveling in a two-dimensional medium such as a water wave traveling through ocean water? Or what if the wave is traveling in a three-dimensional medium such as a sound wave or a light wave traveling through air? What types of behaviors can be expected of such two- and three-dimensional waves? Specifically, there will be some reflection off the boundary and some transmission into the new medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium. The wave doesn't just stop when it reaches the end of the medium. Previously in Lesson 3, the behavior of waves traveling along a rope from a more dense medium to a less dense medium (and vice versa) was discussed.
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