Lissajous figures are sometimes used for the measurement of phase. They are produced in an oscilloscope
by connecting one signal to the vertical trace and the other to the horizontal trace. If the ratio of the
first frequency to the second is a rational number (i.e., it is equal to one small integer divided by another),
then a closed curve will be observed on the CRO (see Figures 41.4 and 41.5). If the two frequencies are
unrelated, then there will be only a patch of light observed because of the persistance of the oscilloscope
screen