ABSTRACT
The electron microscope (Fig. 7.1) is analogous to the light microscope (Chapter 2) in principle. The solid ground glass lenses present in the light microscope are replaced by magnetic field lenses in the electron microscope: the light source of the light microscope is replaced by an electron beam in an electron microscope (Fig. 7.2). The wavelength of visible light (white light) is 540 nm and that of ultraviolet is 260 nm, whereas the wavelength of the beam of electron is usually less than 0.1 nm, depending on the accelerating voltage. The wavelength of electrons in an electron microscope using 60 kV will be about 0.005 nm. The resolving power of the microscope is inversely proportional to the wavelength of light being used. This is the reason for the high resolving power of the electron microscope, which can magnify objects about 200,000 times, whereas even the best light microscope may magnify only up to 1500 times. Electron microscopes that are now available can be used to distinguish objects 1 nm apart with sufficient density to hold back electrons. The image of the object is focused on a fluorescent screen, photographed, and then enlarged for detailed examination. The image of the object cannot be viewed directly as in the case of light microscope. Because of the high resolving power of the electron microscope, it has been possible to study the ultrastructure of biological materials such as individual cells and molecules such as proteins and nucleic acids whose structure had been known only through biochemical analyses.
