Page 42 - Electrical Properties of Materials

The electron as a wave

2

The old order changeth, yielding place to new.
Tennyson The Idylls of the King
Vezess uj utakra, Lucifer.
Madách Az Ember Tragédiája

2.1 Introduction
We have considered the electron as a particle and managed to explain suc-
cessfully 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 proper-
ties too. The chief immigrant in this particular woodpile, the experiment that
could not possibly be explained by a particle model, was the electron diffrac-
∗
tion experiment of Davisson and Germer in 1927. The electrons behaved as ∗ Davisson received the Nobel Prize in
waves. 1937.
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 bul-
lets 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 pat-
tern on the target screen, shown graphically in Fig. 2.1 as a plot of probability

Screen Target
with holes screen

P 1
P = P + P
12 1 2
A

Gun B

P
2
Fig. 2.1
An experiment with bullets.