What happens to the reflected sound at a smooth and flat interface?

When sound waves encounter a smooth and flat interface, such as the boundary between two different media (for example, between water and air or between soft tissue and a dense structure), a significant portion of the sound waves is reflected back into the original medium. This occurs due to the impedance mismatch between the two media.

In clinical practice, such as ophthalmic ultrasound, this principle is crucial because it allows for the visualization of structures within the eye. The smoothness and flatness of the interface facilitate the reflection of sound, enabling the sonographer to gather useful information about the underlying anatomy. The return of most of the sound indicates that the interface is effectively reflecting sound, which is essential for producing clear and accurate imaging.

Thus, the correct answer reflects a fundamental principle of ultrasound physics that plays a vital role in diagnostic imaging techniques.