Scanning Electron Microscopes (SEMs) have astonishingly high magnifications of up to 250,000 X and higher. Optical microscopes are generally limited to magnifications of up to about 5,000 X, and the best normally achievable resolution is of the order of 0.2 microns (200 nm). SEMs also have much greater depth of field than optical microscopes and consequently provide stunning 3D-like images. One other advantage of SEMs is that the instruments can be used to look below the surface of an object, hence revealing some detail about its internal structure.
Unfortunately, SEMs are also very bulky and expensive instruments (typically costing hundreds of thousands of pounds or more). They are also complex and must be used by operators with significant expertise. Their usage is normally limited to specialist industrial and research applications.
An SEM uses a focused beam of high-energy electrons which generates a range of signals at the surface of a solid object. Kinetic energy is dissipated when the electrons are decelerated at the surface of the object under examination, and secondary signals are used to compute a two-dimensional spatial image that reveals information such as chemical composition, crystalline structure and texture. Analysis of a specimen using an SEM is regarded as non-destructive because the electron beams do case any loss of volume in the specimen. The analysis process can therefore be repeated numerous times.
A couple of extraordinary example high-magnification images follow.
Pollens: Picture released into public domain by Dartmouth Electron Microscope Facility at Dartmouth College.
Staphylococcus aureus: Public domain image by Janice Haney Carr - magnification 10,000X