To shorten the focal length of a curved lens, you should:

• A. Increase radius of curvature
• B. Decrease radius of curvature
• C. Increase transducer frequency
• D. Use water path adjustments

Answer: (B)

Shorter focal length comes from stronger bending of the sound wave, which is controlled by the lens’s curvature. When you use a smaller radius of curvature, the lens bends waves more sharply, so the point where the waves converge (the focus) moves closer to the lens. Conversely, increasing the radius makes the lens flatter, reducing bending and pushing the focus farther away.

Changing transducer frequency doesn’t change the geometric focal length because it mainly affects wavelength and resolution, not where the lens focuses. Water path adjustments can alter where the focus appears in practice, but they don’t change the lens’s intrinsic focal length set by its curvature.