Transmission electron microscopy is an analytical imaging technique whereby a beam of electrons are focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope).
In the past, light microscopes have been used mostly for imaging due to their relative ease of use. However, the maximum resolution that one can image is determined by the wavelength of the photons that are being used to probe the sample; nothing smaller than the wavelength being used can be resolved. Visible light has wavelengths of 400-700 nanometers; larger than many objects of interest. Ultraviolet could be used, but soon runs into problems of absorption. Even shorter wavelength, such as X-rays exhibit the problem of their lack of interaction: both in focussing (what can be used for a lens?) and in actually interacting with the sample.
Like all matter, electrons have both wave and particle properties (as demonstrated by Louis-Victor de Broglie), and their wave-like properties mean that a beam of electrons can in some circumstances be made to behave like a beam of radiation. The wavelength is dependent on their energy, and so can can be tuned by adjustment of accelerating fields, and can be much smaller than that of light , yet they can still interact with the sample due to their electrical charge. Electrons are generated by a process known as thermionic discharge in the same manner as the at the cathode in a cathode ray tube; they are then accelerated by an electric field and focussed by electrical and magnetic fields on to the sample. The electrons can be focused onto the sample providing a resolution far better than is possible with light microscopes, and with improved depth of vision. Details of a sample can be enhanced in light microscopy by the use of stains; similarly with electron microscopy, compounds of heavy metals such as lead or uranium can be used to selectively deposit heavy atoms in the sample and enhance structural detail, the dense electron clouds of the heavy atoms interacting strongly with the electron beam.
The electrons can be detected using a photographic film, or fluorescent screen among other technologies. Please visit the external links for more information on transmission electron microscopy.