Problem 5.152

A beam of X-rays with wavelength \(\lambda=40\) pm falls normally on a plane rectangular array of scattering centres and produces a system of diffraction maxima (Fig. 5.29 ) on a plane screen removed from the array by a distance \(l=10 \mathrm{~cm} .\) 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 \(\Delta x=60 \mathrm{~mm}\) (along the \(x\) axis) and \(\Delta y=40 \mathrm{~mm}\) (along the \(y\) axis).