Which zone refers to the far-field region where the beam diverges after the focal point?

• A. Fresnel zone
• B. Focal zone
• C. Near-field zone
• D. Fraunhofer zone

Answer: (D)

The question is testing knowledge of diffraction zones, specifically the far-field behavior after focusing. After light passes the focal point, it spreads out, and at sufficiently large distances the wavefronts become effectively planar. In this far-field region, called the Fraunhofer zone, the diffraction pattern is governed by the angular distribution and is described by the Fourier transform of the aperture. This is distinct from the Fresnel (near-field) zone, where the wavefront curvature and distance cause the pattern to depend strongly on how far you are from the aperture. The term “Focal zone” isn’t a standard designation for a diffraction regime. So, the correct zone is the Fraunhofer zone.