Electron Microscopy is a type of imaging which uses a beam of electrons to illuminate a specimen and create a magnified image of it. This type of microscope has a higher resolving power than that of a light microscope, and therefore can achieve magnifications of objects thousands of times smaller due to the wavelength of an electron being smaller then that of visible light.
The electron Microscope works by a filament/cathode producing a beam of electrons which are then focused by magnetic coils. The electrons are then either scattered or focused to produce an image on a phosphorescent screen or recorded on a photographic plate or high-resolution camera which is then interpreted digitally by a computer.
Scanning Electron Microscopes (SEM) produce a 3D images (including shadowing and highlights) by interpreting electrons that have been scattered or emitted by the surface of the specimen. Specimens are prepared either by rapid freezing where they can be examined directly or by fixing, drying then staining with electron-dense heavy metals such as gold. Little preparation is required and whole plants can be scanned although only images of the cell surfaces are produced so organelles can't be seen in. Magnification of up to 20000x can be produced.
Transmission Electron Microscopes (TEM) as well as producing images of the surface can also, due to their higher resolution than SEM, produce images of macromolecules. Specimens are prepared by being dried then having heavy (electron dense) metals sprayed on at an angle so to deposit the metal thicker in some places than others. If internal images of organelles is wanted then the specimen is prepared by rapid freezing, cracked open with a knife blade and freeze-dried, this exposes parts of the cell which can then be sprayed with the heavy metal before the organic cell tissue is dissolved away leaving a replica of the surface.