The end of the near zone coincides with the location of the

• A. Transducer
• B. Far field
• C. Focus
• D. Center of the crystal

Answer: (C)

In ultrasound, the near field (Fresnel zone) is the region where waves from different parts of the transducer are still converging and the beam is changing shape due to diffraction. The point where the beam narrows to its smallest diameter—the convergence point—is the focus. At the focus, the wavefronts meet and the intensity is highest; beyond that point, diffraction causes the beam to widen into the far field. So the end of the near field lines up with the focus because that is precisely where the beam is most tightly concentrated before it begins to spread again. The transducer itself is the source, not the boundary; the far field starts after the focus; and the center of the crystal isn’t the boundary for the near-field region.