X-rays of wavelength 0.138 nm fall on a crystal whose atoms, lying in planes, are spaced 0.285 nm apart. At what angle $\phi$ (relative to the surface, Fig. 25–38) must the X-rays be directed if the first diffraction maximum is to be observed?
Figure 25-38 X-ray diffraction by a crystal.
Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.