What term describes the direction of the incident beam relative to the media boundary?

• A. Incident angle
• B. Oblique incidence
• C. Perpendicular incidence
• D. Reflected angle

Answer: (A)

In ultrasound, the direction of the incoming beam relative to a boundary is described by the angle of incidence. This is defined as the angle between the incoming ray and the line that is perpendicular (the normal) to the boundary. It covers both straight-on and angled approaches: if the beam hits the boundary directly, the angle is zero (normal incidence); if it comes in at an angle, it’s oblique incidence. The term for the direction of the reflected beam is the reflected angle, which is a different quantity. Perpendicular incidence is simply the zero-angle case of the angle of incidence, not a separate description. So the best term for the incident beam’s direction is the angle of incidence.