Comparing the Light and Electron Microscope

So in 1933 the Electron microscope was developed. The electron microscope works on the same principles as the light microscope but instead of light rays, with their wavelengths in the order of 500nm, a beam of electrons of wavelengths 0.005nm is used. This means that the electron microscope can magnify up to 500,000 times compared to the best light microscopes which magnify only around 2000 times. Whereas the light microscope uses glass lenses to focus the light rays, the electron beam of the electron microscope is focused by powerful electromagnets. The image produced by the electron microscope cannot be detected directly by the naked eye. The beam of electrons is directed on to a screen from which black and white photographs, called Photoelectron micrographs, can be taken. There are two main types of electron microscope. These are called the Transmission electron microscope (TEM), and the Scanning electron microscope (SEM). In the TEM, a beam of electrons is passed through thin, specially prepared slices of material. As the molecules in the air would absorb the electrons, a vacuum has to be created within the instrument. Where electrons are absorbed by the material, and do not therefore reach the screen, the image is dark. Such areas are said to be electron dense. Where electrons penetrate, the screen appears bright. These areas are termed electron transparent. As electrons have a very small mass, they do not easily penetrate materials and so the sections need to be exceedingly thin. This sectioning creates a flat image and the natural contouring of a specimen cannot be seen, to overcome this, the SEM was developed. In the SEM a fine beam of electrons is passed to and fro across the specimen, beginning at one end and working across to the other. The specimen scatters many electrons, while others are absorbed. Low energy secondary ones are amplified and transmitted to a screen. The resultant image shows holes and depressions as dark areas, ridges, and extensions of the surface as bright areas. In this way the natural contouring of the material may be observed.

Below is a table of comparison, of the advantages and disadvantages of the light and electron microscope.

However there are problems with both these forms of electron microscope, it is that it requires complex preparation techniques coupled with the need for a high vacuum means that not only must the material being observed be dehydrated and therefore dead, it is frequently considerably distorted; what you see may be very different from the original material. In response to this a new generation of electron microscopes has been developed-the Environment scanning electron microscope (ESEM). These microscopes allow the material on view to be kept at a low vacuum while the region around the electron gun is at a high vacuum. This is achieved by separating the microscope column into a series of chambers. Each of these chambers has its own pressure. There is only a minute hole between chambers; the hole is wide enough to allow the tiny electron beam through. A special low voltage detector which can operate at low vacuums is used to detect the scattered electrons, secondary electrons and X-rays which provide the image.