2.1 Introduction

We have considered the electron as a particle and managed to explain successfully a number of interesting phenomena. Can we explain the rest of the electronics by gentle modifications of this model? Unfortunately (for students if not lecturers), the answer is no. The experimental results on specific heat have already warned us that something is wrong with our particles, but the situation is, in fact, a lot worse. We find that the electron has wavelike properties too. The chief immigrant in this particular woodpile, the experiment that could not possibly be explained by a particle model, was the electron diffraction experiment of Davisson and Germer in 1927. The electrons behaved as waves.

We shall return to the experiment a little later; let us see first what the basic difference is between particle and wave behaviour. The difference can best be illustrated by the following 'thought' experiment. Suppose we were to fire bullets at a bullet-proof screen with two slits in it (Fig. 2.1). We will suppose that the gun barrel is old and worn, so that the bullets bespatter the screen around the slits uniformly after a fairly large number of shots. If at first, slit B is closed by a bullet-proof cover, the bullets going through A make a probability pattern on the target screen, shown graphically in Fig. 2.1 as a plot...