Rayleigh waves are generated at which critical angle?

• A. Are generated at the second critical angle
• B. Are generated at the first critical angle
• C. Travel only in a liquid
• D. Are another name for Lamb waves

Answer: (A)

Rayleigh waves are surface waves, meaning they travel along the free surface of a solid and decay in depth. In ultrasonic testing with a wedge or immersion setup, energy is launched into the solid at a specific incidence angle. At the second critical angle, the energy couples into a surface-bound mode that propagates along the surface rather than into the bulk—this is the Rayleigh wave. The first critical angle corresponds to the onset of other bulk modes, so it does not produce the surface-bound Rayleigh wave. Lamb waves, on the other hand, are plate waves, not Rayleigh waves on a free surface, and Rayleigh waves are not limited to liquids.