Final answer:
Refraction describes the change in direction of sound or light when it passes between different media; this occurs due to changes in wave speed and is explained by Huygens's principle. Sound waves maintain their frequency but change speed and wavelength, while light conforms to Snell's law, depending on the media's refractive index.
Step-by-step explanation:
The change in direction of sound or light upon entering a different medium can be attributed to a phenomenon known as refraction. Sound and light are both waves that can travel through various media, such as air, water, and solids. According to Huygens's principle, a wavefront encountering a boundary between two different media will produce wavelets, the propagation of which depends on the speed of sound in the respective media. In the case of sound, as it passes from one medium to another, its speed changes due to differences in density and elastic properties of the media. As a result, the direction in which the molecules propagate changes, which is manifested as a change in the wave's direction, or refraction.
When examining how the refraction of sound can be explained at the atomic level, it's essential to understand that sound vibrations differ from thermal motion. Sound vibrations are organized oscillations caused by a propagating wave, whereas thermal motion is the random movement of atoms and molecules due to temperature. When sound moves across the boundary into a different medium, unlike thermal motion, its frequency remains unchanged, but its wavelength may vary depending on the new medium's speed of sound. Therefore, a higher-density medium will generally cause sound to travel slower, leading to a change in the angle of propagation as per Snell's law.
The light case is similar; as it moves from one material to another, its speed changes due to differences in the optical density of the media, denoted by an index of refraction. Light's ray moves closer to the perpendicular when moving from a medium with a lower to a higher index of refraction and vice versa, as shown in Figure 1.13. For sound waves, such changes in the speed and subsequent bending of the wavefront are the reason behind the change in direction of propagation when transitioning between different media.