What does "refraction" refer to in ultrasonic testing?

Refraction in ultrasonic testing refers to the change in direction of a sound wave as it passes from one medium to another with a different acoustic impedance. This phenomenon occurs due to the difference in density and elasticity between the materials. When a sound wave moves from one medium, such as air, into another medium, like steel, its speed and direction will change according to Snell's Law, which describes how waves bend at the interface between two different media. Understanding refraction is crucial in ultrasonic testing because it affects how the sound waves propagate and interact with the material being inspected, influencing the accuracy and reliability of the test results.