Electron Microscopes
In 1924 the French physicist Louis de Broglie suggested that electron beams might be regarded as a form of wave motion, similar to light. Furthermore, he reasoned that the actual wavelength of such a beam would be much shorter than that of a beam of light.
The first commercial electron microscopes were built in the 1930s. The electron microscope is so named because it directs a beam of electrons rather than light through a specimen. The beam of electrons is created in a hot tungsten filament in an electron gun. This beam then travels through the length of the microscope cylinder, which houses the lenses, the specimen chamber, and the image-recording system. Two types of electron lenses are used, electrostatic and electromagnetic. They create electric and electromagnetic fields to both concentrate and move the beam.
The electron microscope requires that the electron beam be in a vacuum, because electrons cannot travel far in air at atmospheric pressure. The column and specimen chamber of the electron microscope are evacuated by pumps. Living specimens cannot be examined with an electron microscope, since they will not survive in a vacuum.
The magnification in magnetic electron microscopes is determined by the strength of the current passing through the electric and electromagnetic lens coils. The image is focused by changing the current through the objective lens coil. In the optical microscope the image is determined by absorption of light by the specimen; in the electron microscope the image results from a scattering of electrons by atoms of the specimen. Since an atom with a high atomic number scatters electrons more than does a light atom, it appears darker. As the beam passes through a specimen, each tiny variation in the structure of the specimen causes a variation in the electron stream. The image produced is then projected onto a fluorescent screen or recorded on film. The electron microscope, with its tremendous resolving power, can magnify specimens over 50,000 times.
Transmission electron microscopes.
The transmission electron microscope is used to observe thin slices of a specimen. It has an electron gun and condenser lens system, which create and concentrate an electron beam. The electrons pass through an objective lens before reaching the specimen on the movable stage. Intermediate and projector lenses focus the electrons passing through the specimen to form an electron image. The image-recording system converts the electron image into a form that is perceptible to the human eye.
Scanning electron microscope.
The scanning electron microscope reveals the surface structure or topography of objects directly. Like the transmission electron microscope, it has an electron gun, condensers, and objectives. Its extremely narrow beam of focused electrons moves over, or scans, the specimen. Two types of electrons--backscattered and secondary--are emitted from the surface of the specimen. Each type has its own detector. Backscattered electrons move in straight lines, whereas secondary electrons move in curved paths. The emission of secondary electrons allows for the fine detailing of electron micrographs.
