What condition must be met for refraction to occur?

For refraction to occur, there needs to be oblique incidence, meaning that the sound wave strikes the boundary between two different media at an angle rather than directly perpendicular to the surface. This angle of incidence is crucial because refraction describes how the path of the wave changes direction as it passes from one medium to another that has a different sound velocity.

A change in impedance is also important, as it indicates that the properties of the materials involved differ, allowing the wave to refract. When sound travels from one medium into another with a different density or elasticity, the speed of sound changes, and this is what causes refraction. The degree to which the wave bends is described by Snell's law, which relates the angles of incidence and transmission to the velocities in the two media.

In contrast, perpendicular incidence would not produce refraction, as the wave would not change direction; it would simply transmit straight through the boundary without any bending. This is why options involving perpendicular incidence do not fulfill the requirements for refraction. Similarly, without oblique incidence, the change in velocity alone wouldn’t lead to refraction, as there is no angled interaction between the two materials.