The region immediately beyond the near field is the far zone.

• A. far zone
• B. focus
• C. near zone
• D. penetration depth

Answer: (A)

In ultrasound beam physics, the beam unfolds from the transducer through two regions: the near field (Fresnel zone) where the wavefronts are still evolving and interference patterns are complex, and the far field (Fraunhofer zone) where the beam has spread out into a more stable pattern with a defined angular distribution. The boundary is not a hard line, but it lies at a distance roughly equal to the square of the aperture diameter divided by the wavelength (d^2/λ). The region immediately beyond the near field is the far zone, because that is the area where the beam has transitioned into the far-field behavior. The focus is a specific point where the beam is intentionally narrow, not the general region beyond the near field, and penetration depth refers to how deep the sound can travel in tissue, not to beam geometry.