2 - The transmission electron microscope

Summary

Introduction

This chapter describes briefly the basic construction and characteristics of the modern transmission electron microscope and discusses its principal modes of operation. Because the electron microscope is an analogue of the optical (or light) microscope, we also consider briefly the basic features of the optical microscope; this will also provide a link with our earlier discussion of the optical principles of image formation by a lens.

The optical microscope

The basic components of an optical microscope for viewing a transparent object are shown in Figure 2.1. We need not be concerned here with the details of the illumination system, other than to note that although the object in the diagram is illuminated with parallel light, the light need not necessarily be parallel in practice. The object O is located outside the front focal plane of the objective lens that forms the first (or intermediate) image I₁ which is real and inverted; the relevant ray diagram is shown in Figure 1.1(a). The image I₁ becomes the object for the eyepiece. The eyepiece is located so that I₁ is behind the front focal plane of this lens, which forms an image I₂ that is observed by the eye; the relevant ray diagram is shown in Figure 1.1(b). I₂ is a virtual and erect image of I₁, and so the final image is inverted with respect to the object.

The objective lens ultimately determines the performance of the microscope.