Smaller diameter crystals produce beams that diverge more in which region?

• A. Near field
• B. Mid field
• C. Deep far zone
• D. Focal zone

Answer: (C)

Divergence from an aperture is governed by diffraction, and the amount the beam spreads is set by the aperture size. In the far-field, the diffraction pattern becomes stable and the spread is roughly theta ≈ λ / D, where D is the diameter. So, when the crystal (the aperture) is smaller, the diffraction angle becomes larger, and the beam spreads more as it propagates in the far-field.

The near-field region doesn’t show the simple, predictable diffraction broadening until you get far enough away; patterns there are more complex and not described by the simple lambda over D relation. The focal zone is not the region where the natural propagation divergence from a small aperture is described.

Thus, smaller diameter crystals produce beams that diverge more in the deep far zone.