Oblique incidence occurs when the incident sound beam strikes the boundary at _______.

• A. 90 degrees
• B. Any angle other than 90 degrees
• C. 0 degrees
• D. 180 degrees

Answer: (B)

Oblique incidence means the ultrasound beam hits the boundary at an angle that is not perpendicular to the boundary’s surface. When the beam is perpendicular (90 degrees to the boundary), that’s normal incidence. So oblique incidence includes any other angle, which is why the correct statement is that the beam strikes at any angle other than 90 degrees. At oblique angles, you’ll see reflection and refraction behaviors described by the usual laws (reflection angle equals incidence angle relative to the normal, and refraction follows Snell’s law), whereas normal incidence has its own simpler behavior with respect to direction change.