The angle formed by an ultrasonic wave as it enters a medium of different velocity than the one from which it came and a line drawn perpendicular to the interface between the two media is called the angle of

• A. Refraction
• B. Incidence
• C. Rarefaction
• D. Reflection

Answer: (B)

When a ultrasonic beam meets a boundary where the speed changes, the angle is defined with respect to the line drawn perpendicular to the interface—the normal. This angle between the incoming beam and the normal is the angle of incidence. It describes how the beam approaches the boundary, before it may partly reflect and partly refract into the second medium. The angle of refraction would be the angle of the transmitted beam with respect to the normal. The other terms correspond to the transmitted bend (refraction), the wave’s pressure variation inside the cycle (rare faction), or the beam bouncing back (reflection). So the description fits the angle of incidence.