As frequency increases in ultrasonic testing, the length of the Fresnel zone ...?

• A. Stays the same
• B. Increases
• C. Decreases
• D. Becomes undefined

Answer: (B)

In ultrasonic testing, the Fresnel (near-field) zone length depends on the transducer’s aperture and the wavelength of the sound. For a fixed aperture, the boundary between near-field and far-field is governed by a relation that scales with D^2/λ. Since the wavelength λ is inversely related to frequency (λ = c/f), increasing frequency reduces λ, and therefore the near-field length increases. In other words, higher frequency makes the Fresnel zone extend farther into the material before the beam settles into the far field. The other options don’t fit this relationship, as the near-field length would not stay the same, decrease, or become undefined with a simple change in frequency.