Introduction

The purpose of any kind of microscope is to form a magnified image of an object whose fine structure cannot be clearly discerned by the unaided eye. In the conventional optical microscope this magnification is achieved by means of two or more glass lenses. The basic characteristic of a lens that enables it to form a magnified image is its ability to bring to a focus a broad beam of light falling on it. Since many of the optical principles of image formation are also applicable to an electron microscope (in which electron beams are focused by magnetic lenses), it is appropriate that we examine these principles in some detail.

Elementary concepts of image formation by a thin lens

We begin by considering a thin double-convex lens with spherical surfaces, such as that shown in Figure 1.1. If a set of light rays parallel to the principal axis is incident upon this lens, Figure 1.1(a), then refraction at the lens surfaces will cause the rays to converge to a point F, called the focal point. Because the rays from a distant object are essentially parallel, the focal point F is the image of an object on the principal axis at infinity. The distance from F to the center of the lens is the focal length f.