There are two basic models of the electron microscopes: Scanning electron microscopes (SEM) and transmission electron microscopes (TEM). In a SEM, the secondary electrons produced by the specimen are detected to generate an image that contains topological features of the specimen. The image in a TEM, on the other hand, is generated by the electrons that have transmitted through a thin specimen. Let us see how these two microscopes work and what kind of information they can provide:

Scanning electron microscope

Figure 18.1 shows a simplified schematic diagram of a SEM. The electrons produced by the electron gun are guided and focused by the magnetic lenses on the specimen.

Figure 18.1 A simplified schematic diagram of a scanning electron microscope.