ABSTRACT
Waves are a common phenomenon in nature. By general definition, a wave is a movement with a certain periodic back-and-forth and/or up-and-down motion. A wave can also be defined as a disturbance that spreads in matter or space, obeying a certain “wave equation.” Among the many types of waves, those that we are familiar with are sound waves, light waves, radio waves, and water surface waves. Although light waves and radio waves belong to electromagnetic waves that can travel in a vacuum, other types of waves need a medium to transmit the disturbance. Waves that propagate through media, depending on the form of wave
transmission, may be divided into two major categories, namely body waves andsurfacewaves. Inthecaseofbodywaves, theycantravel inasinglemedium. A typical example is soundwaves, which are caused by a pressure disturbance in a solid, liquid, or gas. Wave propagation is accomplished by the consecutive compressionanddilatation thatoccur in thewavepropagationdirection. Another example of such a wave is the P-type seismic wave. Besides the above pressure-induced body waves, there are also stress-induced body waves, in which themediummoves in a direction perpendicular to the direction ofwave propagation, for example, S-type seismic waves. Surface waves always appear on the interface of two different media. The
restoring force, which attempts to restore the interface to the equilibrium state, plays a predominant role in wave propagation. Surface waves can occur at the interface between a solid and a fluid, e.g., Rayleigh-type seismic waves. They can also occur at a fluid-fluid interface, e.g., ocean internal waves between two fluid layers with different density and ocean surface waves on an air-water interface.
