How can an oscilloscope be used to calculate the frequency of an alternating current?

In an AC from one peak to the next peak, the current has changed direction and change again so the time from one peak to the next peak is one complete cycle, the time measured between the two peaks is the time it takes for the current to change direction and change back. As the frequency is the number of cycles in one second, it can be calculated by dividing one by the time intervals of the first and second peak.