A student stands some distance from a large wall and claps their hands once. A short time later, the student hears a second sound. Explain why the student hears a second sound and describe the nature of the wave responsible for both sounds.

The sound produced by the clap propagates as a pressure wave through the air. It reaches the wall, where part of the wave is reflected back towards the student. The reflected wave travels a longer path (to the wall and back) and therefore takes more time to arrive, so the student hears a second sound after a short delay.

Both sounds are sound waves, which are longitudinal waves: the air particles oscillate parallel to the direction of wave propagation, creating alternating compressions and rarefactions that carry the energy through the medium.