The angle of reflection at an aluminum–water interface is:

• A. Equal to the angle of incidence
• B. 0.256 times the angle of incidence
• C. Approximately 1/2 the angle of incidence
• D. Approximately 4 times the angle of incidence

Answer: (A)

The angle of reflection follows the law of reflection: the reflected beam leaves at the same angle to the normal as the incident beam arrives, measured at the boundary between the two media. This is true for a smooth, planar aluminum–water interface, where the reflection is specular. The materials’ acoustic impedances affect how much energy is reflected versus transmitted, but they do not change the direction of the reflected wave. So if the ultrasonic wave hits the interface at a given angle, the reflected wave travels at that same angle relative to the normal. If the boundary were rough, you’d get diffuse scattering, but the specular component still adheres to equal incident and reflection angles.