Near field length is also known as which zone?

• A. A Far field
• B. B Fresnel zone
• C. C Near field
• D. D Far-field zone

Answer: (B)

Near-field length is the region where the beam is still forming and the wavefronts from different parts of the transducer aperture interfere to create a changing, nonuniform pattern. This is known as the Fresnel zone. The name comes from diffraction theory and distinguishes this evolving near-field behavior from the more stable far field. The boundary to the far field is typically around D^2/(4λ) for a circular aperture, with D being the transducer diameter and λ the wavelength. Beyond that distance, you’re in the far-field (Fraunhofer zone), where the beam becomes more uniform and the wavefronts are approximately planar. The other terms describe either the far-field region or the general near-field concept, but the standard zone name for the near-field region is the Fresnel zone.