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Work Step by Step
This poetic description of waves is true, as you can verify yourself with a bowl of water, but poem does not address the fact that the initial wave then reflects from the walls (or passes through the center), becoming the other kind of wave. Furthermore, the amplitude of the waves also changes as the waves travel. See the discussion in Section 11–9 for spherical waves in 3-D. In 2-D, as the circular wave expands from the center, the energy it carries is distributed over a larger and larger circumference, so the wave intensity has to decrease. Because the wave intensity is proportional to the square of the amplitude, the amplitude decreases. On the other hand, for waves propagating toward the center, the amplitude increases.