PlanetPhysics/Electron Microscope

An electron microscope (EM) is a fairly large and expensive scientific instrument that employs high voltage to accelerate an electron beam, and also utilizes magnetic fields in order to form a tightly focused beam of accelerated electrons that are then illuminating an area of a specimen placed at the electron beam focus in order to create a million times magnified image of the selected specimen area. The resolution of the electron microscope is therefore determined by both the associated wavelength of the accelerated electrons and by the properties of the specimen, such as its thickness and electron density. The electron microscope uses electrostatic and electromagnetic ``lenses" to control the electron beam and focus it to form an image. The electron beam is first diffracted by the specimen, and then, the electron microscope ``lenses" re-focus the beam into a Fourier-transformed image of the diffraction pattern for the selected area of investigation. The real image thus formed is `magnified' image by a factor of several million, and can be then recorded on a special photographic plate, or viewed on a detecting screen. Its advantages over X-ray crystallography are that the specimen need not be a single crystal or even a polycrystalline powder, and also that the Fourier transform reconstruction of the object's magnified structure occurs physically and thus avoids the phase problem faced by the X-ray crystalographers after obatining the X-ray diffraction patterns of a single crystal or polycristalline powder. Its disadvantage is the need for extremely thin sections of the specimens, typically less than 10 nanometers. For biological specimens it also requires sample fixation treatments and special `staining' with heavy atom labels to achieve the required contrast.