If an ultrasound beam strikes a boundary at normal incidence and both media have the same propagation speed, which process cannot occur?

• A. Reflection
• B. Refraction
• C. Transmission
• D. Attenuation

Answer: (B)

Refraction requires a change in the direction of the wave as it crosses a boundary, which happens when the wave enters at an angle rather than perpendicular to the boundary. At normal incidence, the beam hits the boundary head-on, so it continues straight into the second medium. If the propagation speeds are the same in both media, there’s no tendency for the wave to bend at all, so refraction cannot occur.

However, reflection can still happen due to boundary conditions such as impedance differences, and the wave can transmit into the second medium along the same line. Attenuation can also occur as the wave propagates and loses energy within the media.