29.1

The Wave-Particle Duality

The ability to exhibit interference effects is an essential characteristic of waves. For instance, Section 27.2 discusses Young’s famous experiment in which light passes through two closely spaced slits and produces a pattern of bright and dark fringes on a screen (see Figure 27-4). The fringe pattern is a direct indication that interference is occurring between the light waves coming from each slit.
One of the most incredible discoveries of twentieth-century physics is that particles can also behave like waves and exhibit interference effects. For instance, Figure 29-1 shows a version of Young’s experiment performed by directing a beam of electrons onto a double slit. In this experiment, the screen is like a television screen and glows wherever an electron strikes it. Part a of the drawing indicates the pattern that would be seen on the screen if each electron, behaving strictly as a particle, were to pass through one slit or the other and strike the screen. The pattern would consist of an image of each slit. Part b shows the pattern actually observed, which consists of bright and dark fringes, reminiscent of what is obtained when light waves pass through a double slit. The fringe pattern indicates that the electrons are exhibiting the interference effects associated with waves.

Figure 29-1 (a) If electrons behaved as discrete particles with no wave properties, they would pass through one or the other of the two slits and strike the screen, causing it to glow and produce exact images of the slits. (b) In reality, the screen reveals a pattern of bright and dark fringes, similar to the pattern produced when a beam of light is used and interference occurs between the light waves coming from each slit.

But how can electrons behave like waves in the experiment shown in Figure 29-1b? And what kind of waves are they? The answers to these profound questions will be discussed later in this chapter. For the moment, we intend only to emphasize that the concept of an electron as a tiny discrete particle of matter does not account for the fact that the electron can behave as a wave in some circumstances. In other words, the electron exhibits a dual nature, with both particle-like characteristics and wave-like characteristics.
Here is another interesting question: If a particle can exhibit wave-like properties, can waves exhibit particle-like behavior? As the next three sections reveal, the answer is yes. In fact, experiments that demonstrated the particle-like behavior of waves were performed near the beginning of the twentieth century, before the experiments that demonstrated the wave-like properties of the electrons. Scientists now accept the wave-particle duality as an essential part of nature:

> Waves can exhibit particle-like characteristics, and particles can exhibit wave-like characteristics.

Section 29.2 begins the remarkable story of the wave–particle duality by discussing the electromagnetic waves that are radiated by a perfect blackbody. It is appropriate to begin with blackbody radiation, because it provided the first link in the chain of experimental evidence leading to our present understanding of the wave-particle duality.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.