What condition is described by the second critical angle?

The second critical angle refers to a specific condition in the context of wave propagation, particularly in ultrasonic testing. In this scenario, the correct choice indicates that at the second critical angle, the refracted shear wave reaches 90 degrees relative to the boundary between two media.

In more detail, the phenomenon occurs during the interaction between longitudinal and shear waves when the angle of incidence in the longitudinal wave increases. When this angle reaches a critical point, known as the first critical angle, the longitudinal wave is refracted into the shear wave. Continuing to increase the angle of incidence eventually leads to the second critical angle, where the shear wave is refracted at exactly 90 degrees to the normal at the interface. This result is significant because it marks the maximum angle at which the shear wave can propagate within the second medium.

Understanding this concept is crucial for effective ultrasonic testing, as it helps practitioners predict how different angles of incident ultrasonic waves behave when interacting with materials of different properties, ultimately affecting the accuracy of flaw detection and characterization.