If the number of throughs that pass a point in a given time increases, what has decreased

The number of throughs (and crests) that pass a point in a given time gives the frequency of the wave. In fact, the frequency is exactly defined as the number of complete cycles of a wave that pass a given point in a given time. Therefore, if the number of throughs that pass a point in a given time increases, the frequency of the wave increases as well. But the frequency is the reciprocal of the period:

where T is the period. The two quantities are inversely proportional to each other, therefore since the frequency has increased, the period of the wave has decreased.

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