Sine wave is what? The one to have shown the height which changes when moving the circumference in the time is the sine wave. It returns to the original place when going around the circle.At present, it uses the unit, the hertz (Hz), to show the frequency. It was using the word, the cycle per second, before as the unit which shows the frequency.This shows the 1-second number of rotations.(The once is the 1 cycle) The many turns occur in the high frequency.With the figure on the left, 0V positive 0V negative 0V is called the 1 cycle.There is "a period" in the word which is related with the frequency. This is the time which the 1 cycle takes. It becomes the frequency reciprocal.Period(T) = 1 / Frequency(F) and Frequency(F) = 1 / Period(T)In case of Japan, the electricity to be using in the home is 50 Hz or 60 Hz. In case of 50 Hz, the 1-cycle sine wave occurs the 50 times for the 1 second. 60 Hz are the 60 times.Because it is, the 1 period becomes 1/50 = 0.02 seconds (the 20 milliseconds). In case of 60 Hz, it is the 16.67 millisecond.There is a way of making make the Lissajous as the way of understanding that the sine wave relates to the circle. In case of the sine wave which doesn't have the distortion, the Lissajous becomes the true circle.