The angle between the incident sound beam and an imaginary line that is perpendicular to the boundary is called the ______.

• A. Angle of incidence
• B. Angle of reflection
• C. Angle of transmission
• D. Angle of refraction

Answer: (D)

The angle of incidence is defined as the angle between the incoming sound beam and the normal, a line drawn perpendicular to the boundary. This normal is the reference for measuring the incidence angle, and it’s the same line used to define the angle of reflection and the angle of refraction (transmission) in the second medium. So, since the question specifies the angle between the incident beam and the line perpendicular to the boundary, that is the angle of incidence. The angle of refraction, in contrast, refers to the path of the transmitted beam in the second medium and is measured relative to the same normal after crossing the boundary.