Introduction

WBCHSE's Unit 10, "Oscillation and Waves", studies periodic motion in two stages. SUB TOPIC 1 examines simple harmonic motion (SHM) in an isolated oscillating system -- a pendulum, a mass on a spring -- where the disturbance stays confined to that one system. This second sub-topic, Waves, asks what happens when such an oscillation is not confined but is passed on, particle by particle, through an extended medium: the result is a wave, a disturbance that carries energy (and momentum) from one place to another without any net, bulk transport of the medium's own particles -- each particle of the medium merely oscillates, typically in SHM, about its own fixed mean position, while the pattern of disturbance itself travels onward. This chapter builds up the description of such travelling (progressive) mechanical waves -- transverse and longitudinal, the equation describing a travelling wave's displacement, and the factors that fix how fast such a wave moves through a given medium, including the historically important correction Laplace made to Newton's original formula for the speed of sound. It then turns to what happens when two or more waves overlap (the principle of superposition), and works through its three most important consequences for this syllabus: stationary (standing) waves set up by reflection in a stretched string and in the air columns of open and closed organ pipes, the phenomenon of beats produced when two close but unequal frequencies overlap, and the Doppler effect, the shift in a sound's perceived frequency whenever there is relative motion between source and observer.