263
 Unit C
Wave behaviour
INTRODUCTION
In Topic B we discussed one of the great models of Physics, the particle model. In this topic we will meet another great model of Physics, the wave model. Wave behaviour can be found everywhere in nature and is responsible for spectacular phenomena such as the blue colour of the sky and the colours of the rainbow. It is also involved in terrifying phenomena such as earthquakes and tsunamis. Wave behaviour helps us understand the sound from a violin or a flute. Light itself was shown to be a wave in one of the greatest unifications ever made in Physics, Maxwell's electromagnetic theory in the second half of the 19th century.
We will see that, basic to any wave motion is the concept of an oscillation, the back and forth motion such as that of a swing hanging from a tree or the motion of an atom in a metal rod that has been hit on one end. The need to understand wave motion in terms of oscillations led to the development of an enormous field in Mathematics called Fourier analysis. There have been many instances where the need to understand a particular physical phenomenon led to the development of entire branches of Mathematics. Calculus is the obvious example with Fourier analysis a close second.
We will meet many phenomena associated with waves but the one that will stand out will be that of interference: two waves meeting at a point may re-enforce each other or completely cancel each other out. Interference has no analogue in the particle model and so its presence is the revealing sign of wave behavior. So it came as a great surprise in the beginning of the 20th century that electrons also show interference phenomena, implying that electrons have a dual waveâparticle nature. That realisation was the beginning of the revolution of quantum theory. We will discuss a few of these ideas in Topic E.