Walker, Halliday & Resnick — Diffraction: Problem 1

1 In Fig. 36-30, let a beam of x rays of wavelength $0.122\text{ nm}$ be incident on an NaCl crystal at angle $\theta = 45.0^\circ$ to the top face of the crystal and a family of reflecting planes. Let the reflecting planes have separation $d = 0.252\text{ nm}$. The crystal is turned through angle $\phi$ around an axis perpendicular to the plane of the page until these reflecting planes give diffraction maxima. What are the (a) smaller and (b) larger value of $\phi$ if the crystal is turned clockwise and the (c) smaller and (d) larger value of $\phi$ if it is turned counterclockwise?