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

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.

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.