Sine waves repeat over time, so this makes them periodic. The time is takes for a part of the sine wave (one cycle) to travel before it repeats itself is called the period, represented by the letter T. The frequency of a sine wave is the number of oscillations it makes in one second. This is measured in Hertz, which have units of s. The frequency can be found by dividing 1/T (this is also called taking the reciprocal). 