Introduction

Along with interference, diffraction is a manifestation of the wave nature of light and cannot be explained using geometrical optics or ray theory alone. In this chapter, we will explore the nature of diffraction and some applications to visual optical instrumentation, such as the Fresnel zone plate, speckle patterns and Fraunhofer diffraction. In order to explain the theory of these processes fully, we will need to introduce certain aspects of diffraction theory and important equations. The development of some of these equations will be beyond the scope of this book and therefore will be taken from other texts. Unless otherwise stated, the principal source of these equations and the historical development of diffraction will be Born and Wolf (1989).

The nature and cause of diffraction

In a number of situations, it can be observed that when a wave motion passes through an aperture, the waves bend around the edge of the aperture. For example, sea waves on entering an enclosed harbour will bend around the sea wall. Other examples are sound travelling around corners and a number of observable phenomena in the propagation of light. This effect is called diffraction and returning to the above example of sea waves entering a quiet harbour, as shown in Figure 26.1, let us look at what would happen if diffraction did not take place. Let us assume that the waves entering the harbour are parallel. They would remain parallel with a sharp edge at the boundary with the quiet water, producing a vertical wall of water with a sinusoidal profile at the edge as shown in the diagram.