Which phenomenon describes bending of waves at an interface due to a change in propagation speed?

• A. Reflection
• B. Diffraction
• C. Refraction
• D. Diffusion

Answer: (C)

Refraction is the bending of waves when they pass into a medium with a different propagation speed. As the wave crosses the interface, its speed changes, and the direction shifts so that the wavefront remains continuous along the boundary. This change in direction follows Snell’s law: sin of the incident angle divided by the first speed equals sin of the refracted angle divided by the second speed. If the second medium is faster, the beam bends away from the normal; if slower, it bends toward the normal. In ultrasonic testing, refraction explains how angled beams are steered through different materials and interfaces. This is different from reflection, where waves bounce back, or diffraction, where waves bend around edges.