What is an electron microscope for? Why do we have it?
While the smallest objects that your unaided eye can see are about 0.1 mm long, and the light microscope can magnify small objects for you up to 1000 times their original size, the electron microscope can increase magnification another 1000-fold! Consider Fig. 1 below, showing a log scale of various structures resolved by the human eye, by conventional light microscopy or by electron microscopy. With your eye, you can see a grain of salt or even the thickness of a human hair. A light microscope will help you to see most cells (like blood cells, or bakers yeast), and even bacteria. With an electron microscope, there is an even greater range of objects that can be examined with fine detail: from the hairy surface of an insect exoskeleton, to the hairy surface of a cell and its organelles and their intricate interiors, the structure of a virus, macromolecules such as DNA or individual proteins, and even individual atoms of which matter is composed. (For some example EM images, see "What can you see with EM?") This range of dimensions spans seven orders of magnitude, and because of this the electron microscope has profoundly shaped our understanding of anatomy and cell biology.
Figure 1: Resolving power of unaided eye versus light and electron microscopy.