Which angle is associated with the formation of a surface wave at the interface?

• A. First Critical Angle
• B. Second Critical Angle
• C. Third Critical Angle
• D. Zero Degrees

Answer: (B)

When a wave hits an interface between two media, there are angles where the wave in the second medium would travel along the boundary instead of entering deeper—these are the critical angles. In a solid, two wave modes can propagate (longitudinal and shear), so there are two distinct critical angles. The surface wave that travels along the interface forms when the incidence angle matches the condition for the shear-mode wave to skim the boundary, i.e., the second critical angle. At this angle, energy couples into a surface (Rayleigh-type) wave that propagates along the interface. A zero-degree incidence is normal and does not produce a surface wave along the boundary.