The incident ultrasound traveling at an oblique angle to the boundary is called?

• A. Oblique incidence
• B. Circumferential incidence
• C. Normal incidence
• D. Specular incidence

Answer: (A)

Oblique incidence describes a ultrasound ray that meets a boundary at an angle other than perpendicular. When the incidence is oblique, part of the energy reflects and part transmits into the second medium, with the reflected ray emerging at the same angle to the normal as the incident ray (law of reflection) and the transmitted ray bending according to the speeds of sound in the two media (Snell’s law). If the ray were perpendicular to the boundary, that would be normal incidence. Specular incidence relates to how a smooth surface reflects light or sound, not to the angle itself, so it doesn’t describe the situation of hitting the boundary obliquely.