Which sound interaction produces the shadows at curved interfaces?

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

Answer: (A)

The main idea is refraction: when a sound wave crosses a boundary where the speed of sound changes, its path bends. On a curved interface, this bending varies along the curve, so many rays are redirected away from certain forward directions. That creates a region behind the curve where fewer sound waves reach—the shadow. The amount and direction of bending depend on the speeds in the two media (Snell’s law): if the second medium is slower, the ray bends toward the normal; if faster, it bends away. Diffraction involves spreading around edges, scattering is redirection by inhomogeneities, and reflection sends energy back toward the source, none of which explain the directed shadow behind a smooth curved boundary as cleanly as refraction does.