He is measuring the frequency
Garrett is measuring the frequency of the waves, which is the number of waves reaching a point per unit time. Frequency is closely related to the wave's period, and it is measured in hertz (Hz). ;
Garrett is measuring the frequency of the waves that reach the shore over a set time period. Frequency is the number of waves that pass a point in a second, measured in hertz (Hz). For instance, if he counts 5 waves in 10 seconds, the frequency would be 0.5 Hz.
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