Introduction

A wave is a periodic, regular disturbance that travels through (or without) a medium, carrying energy and momentum from one point to another without causing any net flow of the medium's own material. Drop a stone into still water and the ripples spread outward -- the water molecules themselves only bob up and down about their own rest positions, they do not travel outward with the ripple; only the disturbance, and the energy it carries, moves outward.

Waves fall into three broad families. Mechanical waves need a material medium to travel through -- water waves, waves on a stretched string, seismic waves and sound waves are all mechanical. Electromagnetic (EM) waves arise from periodic oscillations of electric and magnetic fields; unlike mechanical waves they can travel through a material medium but do NOT need one, so they can cross vacuum (light is the everyday example; EM waves are studied in a later chapter). Matter waves are the wave associated with any moving object, however massive, and belong to the domain of quantum mechanics.

A travelling or progressive wave is one in which a disturbance created at one place keeps travelling outward to distant points, until some external agency stops it (a wall, a boundary, an obstacle). All the water-ripple examples above are travelling waves. There is a second broad family, stationary (standing) waves, in which the disturbance does not travel onward in this way -- that family is taken up in full only in the next standard (Class XII), and this chapter deliberately restricts itself to travelling/progressive mechanical waves, building up their common properties before specialising to sound.