Incident ultrasound traveling at a perpendicular angle to the boundary is termed?

• A. Perpendicular incidence
• B. Oblique incidence
• C. Normal incidence
• D. Tangential incidence

Answer: (A)

When ultrasound hits a boundary straight on, the beam is oriented perpendicular to the surface. This is described as normal incidence. In this geometry, the angle of incidence relative to the boundary’s normal is zero, so the transmitted wave travels straight into the second medium and the reflection returns along the same path. Some sources also use the term perpendicular incidence to describe this same setup, but the standard ultrasound terminology is normal incidence. The other terms describe oblique or near-tangent approaches, which involve different proportions of reflection and transmission and, in general, refraction.