The frequency, then, is equal to the number of cycles in  one revolution multiplied by the number of revolutions  per second.  The time required for a sine wave to complete one full  cycle is called a period (P). A period is typically measured  in seconds, milliseconds, or microseconds.  [Figure 9-14] The period of a sine wave is inversely proportional to the  frequency. That is to say that the higher the frequency, the  shorter the period. The mathematical relationship between  frequency and period is given as:  The distance that a waveform travels during a period is  commonly referred to as a wavelength and is indicated by the  Greek letter lambda ( λ ). Wavelength is related to frequency  by the formula: The higher the frequency is, the shorter the wavelength is.  The measurement of wavelength is taken from one point  on the waveform to a corresponding point on the next  waveform.  [Figure 9-14]  Since wavelength is a distance,  common units of measure include meters, centimeters,  millimeters, or nanometers. For example, a sound wave of  frequency 20 Hz would have wavelength of 17 meters and  a visible red light wave of 4.3 × 10 –12 Hz would have a  wavelength of roughly 700 nanometers. Keep in mind that  the actual wavelength depends on the media through which  the waveform must travel.  Phase is the relationship between two sine waves, typically  measured in angular degrees. For example, if there are two  different alternators producing power, it would be easy to  compare their individual sine waves and determine their  phase relationship. In  Figure 9-16B , there is a 90° phase  difference between the two voltage waveforms. A phase  relationship can be between any two sine waves. The phase  relationship can be measured between two voltages of  different alternators or the current and voltage produced by  the same alternator. Figure 9-16A  shows a voltage signal and a current signal  superimposed on the same time axis. Notice that when the  voltage increases in the positive alternation that the current  also increases. When the voltage reaches its peak value, so  does the current. Both waveforms then reverse and decrease  back to a zero magnitude, then proceed in the same manner  in the negative direction as they did in the positive direction.  When two waves are exactly in step with each other, they  are said to be in phase. To be in phase, the two waveforms  must go through their maximum and minimum points at the  same time and in the same direction.  Period Defined  Wavelength Defined Phase Relationships Figure 9-16.  In-phase and out-of-phase conditions. Current Voltage 0° 90° 180° 270° 360° Voltage source 1 (leads source 2) 0° 90° 180° 270° 360° Voltage source 2 (lags source 1) Voltage source 1 0°  90° 180°  270° 360° Voltage source 2 A. Voltage and current are in phase B. Two voltage waves, 90° out of phase C. Two voltage waves, 180° out of phase P =  1 f F =  1 P  wave speed  = wavelength  frequency 9-11 Period Frequency