Which scenario could indicate a long, narrow reflection path when beam divergence causes a reflection from a side surface before the back surface?

• A. Multiple indications before the first back reflection
• B. Indications from multiple surface reflections
• C. Conversion from the longitudinal mode to shear mode
• D. Loss of front surface indications

Answer: (C)

Mode conversion at a boundary can cause part of the incident longitudinal wave to become a shear wave when it meets a side surface. Longitudinal and shear waves travel at different speeds and refract at different angles, so the converted shear component follows a distinct, longer path inside the plate. If beam divergence makes energy reflect from a side surface before reaching the back surface, the shear-mode portion travels along that longer route and returns a delayed, narrower echo. This creates a long, narrow reflection path on the indication trace that wouldn’t occur from a simple direct back-surface reflection. Other patterns, such as multiple echoes from different surfaces or near-surface issues, don’t specifically reflect this mode-converted, longer-path behavior.