The angle of refraction for longitudinal waves in the immersion setup is approximately:

• A. 16 degrees
• B. 22 degrees
• C. 18 degrees
• D. 26 degrees

Answer: (A)

In immersion ultrasonic testing, the path change when a longitudinal wave moves from water into a solid is governed by Snell’s law for acoustics: the ratio of the sines of the incidence and refraction angles equals the ratio of the wave speeds in the two media. Water has a much slower wave speed than most solids, so the ratio of speeds c_water to c_solid is small. That means for typical angles used in practice, the refracted angle inside the solid ends up in the lower end of the range, around the mid-teens. Practically, this yields roughly a 16-degree refraction angle for longitudinal waves in common immersion setups. The other options would require less typical incidence conditions or different material properties, so they’re less representative of standard practice.