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A vibration source and a medium are needed to form a mechanical wave.
机械波的形成需要振源和介质。
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The wave continues to grow as it sweeps back and forth across the active medium.
光波在激活媒质中来回穿过时，将不断得到加强。
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The wave continues to grow as it sweeps back and forth across the active medium.
光波在激活媒质中来回穿过时，将不断得到加强。
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