Which term describes ultrasound incidence when the beam hits the boundary at a non-90-degree angle?

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

Answer: (A)

Oblique incidence describes ultrasound reaching a boundary at a non-perpendicular angle. When the beam meets the interface at an angle other than 90 degrees, it is not normal (perpendicular) incidence; the angle is measured relative to the boundary’s normal. At oblique incidence, part of the acoustic energy is reflected and part is transmitted, with the reflected ray forming an angle equal to the incidence angle on the same side of the normal, while the transmitted ray bends according to the speed of sound in the second medium (Snell’s law). This is why the echo strength and direction can change with angle, and why a smooth interface may produce a weaker or differently located echo when the beam is oblique. Normal incidence, by contrast, occurs when the beam is perpendicular to the boundary.