Introduction

In this chapter, we will introduce the concept of an image forming system in its most general sense. By tracing rays from an object through the system, using Snell's law at each surface, we will show how to find the image of that object. When we decide to ray trace, there are two types of rays that we can choose, (a) finite or real rays and (b) paraxial rays. A finite or real ray is a general exact ray, and a paraxial ray is a special type of finite ray that is traced very close to the optical axis. One distinct advantage of paraxial rays is that their ray trace equations are much simpler than finite ray trace equations and hence are easier to apply. In this chapter, we will look at each of these two types and use the paraxial rays to develop a concept of the “ideal” image.

In the next chapter, Chapter 3, we will use the behaviour of paraxial rays to explore some of the properties of both simple and more complex optical systems. We will show that given the details of these properties, we can often find the ideal image positions and sizes without recourse to any type of ray tracing.

Image formation

We define an imaging optical system as a system consisting of any number of refracting or reflecting surfaces. Usually the surfaces will be spherical and we will assume that the centres of curvature of each of the spherical surfaces lie on a single line called the optical axis. Such a system is depicted schematically in Figure 2.1, but without any individual surfaces shown.