The equation giving the condition under which a beam of X-rays will reflect from a crystal lattice with interference maxima. The optical phase difference in the above diagram is
where the is the vacuum wavelength, is the index of refraction as labeled, and d is the distance between layers.
References
Eisberg, R. and Resnick, R. Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles, 2nd ed. New York: Wiley, pp. 58-59, 1985.
