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).