Microscope

Prior to 1930, all microscopes were optical. In 1931, German physicist Ernst Ruska developed the electron microscope that used a beam of moving electrons to illuminate an object instead of light. Magnetic lenses or electric coils produce magnetic fields to deflect the electrons in the same manner that glass lenses bend light rays. The specimen has to be in a vacuum, however, because electrons cannot travel through air. Electron microscopes give point-to-point resolutions of less than 0.2 nanometers. This high resolution permits the direct visualization of many molecules and some atoms.

The transmission electron microscope (TEM) images specimens a fraction of a micrometer or less in thickness. In a TEM, the beam passes through the specimen so that some of the electrons are absorbed and some scattered. The remaining electrons are focused onto a fluorescent screen or special photographic plates via the use of magnetic lenses. The resulting image is in black and white.

In the scanning electron microscope (SEM), a narrowly focused electron beam is scanned over the surface of a solid object and used to build up an image of the details Researcher in biochemistry lab using a transmission electron microscope (TEM). Photograph by R. Maisonneuve. Photo Researchers, Inc. Reproduced by permission. of the surface structure through reflection. Researchers use this type to study minute details on a surface of an object. These microscopes created 3-D images that are magnified up to 50,000x.

Although there are several other special types of electron microscopes, perhaps the most valuable is the electron-probe microanalyzer, which allows a researcher to make a chemical analysis of the composition of materials. This type of microscope uses the incident electron beam to excite the emission of characteristic x radiation by the various elements composing the specimen. Spectrometers built into the instrument detect and analyze the x rays. Viewing the resulting image, the researcher can easily correlate the structure and composition of the material.