Problem 6.60

A narrow stream of monoenergetic electrons falls at an angle of incidence $\theta=30^{\circ}$ on the natural facet of an aluminium single crystal. The distance between the neighbouring crystal planes parallel to that facet is equal to $d=0.20 \mathrm{nm}$. The maximum mirror reflection is observed at a certain accelerating voltage $V_{0} .$ Find $V_{0}$ if the next maximum mirror reflection is known to be observed when the accelerating voltage is increased $\eta=2.25$ times.