Q9Engineering Physics

When a highly monochromatic beam of short-wavelength X-rays physically strikes a regular crystalline solid, the parallel atomic planes systematically act as partial mirrors. For the reflected X-rays to physically undergo absolute constructive interference (producing a massive, detectable intensity peak), their total optical path difference must mathematically exactly equal an integer multiple of the wavelength.

• $d$ = The exact perpendicular interplanar spacing between the crystal lattice planes.
• $\theta$ = The specific glancing angle of incidence (measured strictly from the atomic plane, not the normal).
• $n$ = An integer ($1,2,3\dots$) representing the exact order of diffraction.
• $\lambda$ = The absolute wavelength of the incident X-ray beam.