With normal incidence, what factors affect refraction of ultrasound?

• A. Propagation speeds
• B. Frequencies
• C. Attenuation coefficients
• D. None of the above

Answer: (D)

When a ultrasound wave hits a boundary head-on, it travels along the normal to the surface, so there is no bend in its path. Refraction depends on the change in direction described by Snell’s law, which relates the incidence and transmission angles to the speeds in the two media. If the incidence angle is zero, the transmitted angle must also be zero, meaning the ray continues straight through the boundary regardless of how different the speeds are in the two media.

Thus, propagation speeds, frequency, and attenuation coefficients do not affect refraction at normal incidence. (A boundary mismatch can still cause reflection, but that’s a separate phenomenon from refraction.)