In a disc shaped crystal, the width of the beam at the focus is one half the width of the beam leaving the transducer.

In a disc shaped crystal, the width of the beam at the focus is one half the width of the beam leaving the transducer.

• A. One quarter
• B. One half
• C. Full width
• D. Three quarters

Answer: (B)

When a circular transducer focuses, the beam narrows because waves from all parts of the disc converge to the focal point. For a disc-shaped crystal, a practical rule of thumb is that the beamwidth at the focus becomes about half the width of the transducer itself. This diffraction-limited narrowing happens as the converging wavefronts from opposite edges constructively overlap in the focal region, producing a tight central lobe. The exact width can vary with wavelength and focal length, but the typical outcome is a beam at the focus that is roughly half the transducer’s width, which explains the improved lateral resolution there. The other options don’t reflect this common focusing behavior: the beam isn’t as wide as the transducer at focus (not full width), nor as narrow as a quarter or only modestly narrower than three quarters.

When a circular transducer focuses, the beam narrows because waves from all parts of the disc converge to the focal point. For a disc-shaped crystal, a practical rule of thumb is that the beamwidth at the focus becomes about half the width of the transducer itself. This diffraction-limited narrowing happens as the converging wavefronts from opposite edges constructively overlap in the focal region, producing a tight central lobe. The exact width can vary with wavelength and focal length, but the typical outcome is a beam at the focus that is roughly half the transducer’s width, which explains the improved lateral resolution there. The other options don’t reflect this common focusing behavior: the beam isn’t as wide as the transducer at focus (not full width), nor as narrow as a quarter or only modestly narrower than three quarters.