As with any vibration curve, the amplitude due to any given portion of the wave
front may be obtained by finding the length of the chord of the appropriate segment
of the curve. The square of this length then gives the intensity. Thus the Cornu's
spiral of Fig. I8N can be used for the graphical solution of diffraction problems, as
will be illustrated below. It is to be noted at the start, however, that the numerical
values of intensities computed in this way are relative to a value 0/2/or the unobstructed
wave. Thus, if A represents any amplitude obtained from the plot, the intensity I,
expressed as a fraction of that which would exist if no screen were present, which we
shall call 10, is