At an interface which may occur when ultrasonic wave strikes?

• A. Only reflection occurs
• B. Only refraction occurs
• C. Reflection, Refraction, and Mode conversion can all occur depending upon the incident angle and material velocities
• D. No interaction occurs

Answer: (C)

When an ultrasonic wave reaches an interface between two media, energy can behave in several ways because the two materials typically have different wave speeds and impedances. Part of the energy reflects back into the first medium, and part transmits into the second medium with a change in direction—that’s refraction. In elastic solids, waves can exist as P-waves (compressional) and S-waves (shear), and at an interface the incident wave can generate multiple new waves: reflected and transmitted P-waves, as well as transmitted S-waves (and, if the incident is S, reflected P or S as well). The particular mix and presence of these modes depend on the incidence angle and the velocities (and impedances) of the two media. So you can have reflection, refraction, and mode conversion all occurring under the right conditions. In fluids, only P-waves propagate, but in solids the potential for P-to-S and S-to-P conversion makes all three phenomena possible.