A beam of X-rays with wavelength ? = 40 pm falls nor really on a plane rectangular array of scattering centers and produces a system of diffraction maxima (Fig. 5.29) on a plane screen removed from the array by a distance l = 10 cm. Find the array periods a and b along the x and y axes if the distances between symmetrically located maxima of second order are equal to ?x = 60 mm (along the x axis) and ?y = 40 mm (along the y axis).