Electron microscopes use high-speed electrons in a vacuum to form images of samples. Since electrons have a much smaller wavelength than light, electron microscopes have greater resolution than light microscopes.
The transmission electron microscope is the earliest innovation in electron microscopy. In a transmission electron microscope, a high-voltage beam of electrons passes through a specimen. Some of the electrons pass through the beam while some scatter away. The emerging electrons carry information about the structure of the specimen, and the microscope's objective lens magnifies this information. A viewing screen or photographic plate collects the information as an image.
Scanning electron microscopes use an electron beam to probe and scan a rectangular section of a sample. As the electron beam interacts with the specimen it loses energy as heat, light or X-rays which carry information about the surface structure of the specimen. Because the electrons of a scanning electron microscope scan the surface rather than pass through a specimen, scanning microscopes can image much thicker specimens than a transmission electron microscope, even scanning whole organisms. Scanning electron microscopes yield three-dimensional images of arthropods and bacteria.
The major drawback of electron microscopes is their inability to study living organisms. Light microscopes yield excellent information on the movement of microorganisms, but the necessary vacuum of an electron microscope prevents the study of anything living. Also, transmission electron microscopes often require specimens to be in very thin sections.