The principles and limitations of transmission and scanning electron microscopes.

Transmission electron microscopes (TEM) shine a beam of electrons through a slice of stained specimen, some electrons are absorbed and others travel through; the pattern makes a photomicrograph on a fluorescent screen which records a magnified image of the specimen. The electron beam is manipulated by electromagnets:

Scanning electron microscopes (SEM) shin a beam of electrons on the specimen, but instead of going through they scatter off the surface- the electrons are collected and the pattern amplified to give a 3D image of the specimens surface. SEMs have a very complex system but this diagram shows some of the basic components:

                                                   TEM                SEM

Maximum resolution:                   1nm                  10nm

Maximum magnification:              250000x           100000x

Image:                                        2D                    3D

In both case the specimen must be dead because it is done in a vacuum. If there was air present the electrons would reflect off of it not the specimen.

TEM requires a complex staining process, and for the specimen to be cut up into extremely small pieces so the electrons can get through. Often TEM show up random artefacts (which are areas on the image that don't really exist) which are a result of the way the specimen was prepared.