The optical microscopes are limited in their resolution R because it depends of the wavelength according to the following relation:

with :

n : index of the medium
U: angle of the electron beam (half great point angle)
0,61: coefficient related to the diffraction of Fraunhofer.
: is the wavelength of the radiation
for a radiation of particle of mass m and speed v.
h : Planck's constant

An electron beam has a wavelength much lower than that of a beam of light from where a resolution much smaller.

Moreover, if the electron speed is raised, the wavelength decreases. One obtains practical resolutions provided in the table below.

(photon) = 400 à 700 nm (electron) = 0,001 nm  R (photons) = 500 nm  R (electron) = 0,2 nm

1 nm = m

We can note that the value of the resolution practises electron microscope is not that awaited theoretically because the aperture of the electron beam cannot exceed 1° contrary to the beam of light whose angle u can reach 65°.
We can have a gain of resolution with the electronic microscopes of 1000 X instead of 100 000 X in theory.

Essential qualities of an electron microscope:

- to have a monocinetic beam i.e. which one must be able to regulate the accelerating tension with a 1 V precision.
- good lenses.
- for the electronic microscopes with transmission, a good factor of penetration of the radiation which must be without damage for the object observed.

The transmission electron microscopy principle is closed to the optical microscope. The electronic beam goes throught the sample and loses in the passing a certain number of electrons. The first images were obtained for the first time by Ernst Ruska in 1931.

Principle :

The electron emission is produced by heating of a tungsten filament or a crystal of lanthanum hexaborure.

A high vacuum is carried out in the tube of the microscope.

The accelerating tension is about 200 kV for the cheapest apparatuses and 1000 kV for most expensive (2 $ per volt) !

The magnetic lenses made up of a coil and an iron core, focus the electron beam. The variation of the focal distance makes it possible to vary the enlargement (up to 1 000 000 X) and the focus . The visual observations are always relayed by a photograph catch. It is enough to make rock the screen so that the photographic plates are impressed. The focus does not need to be changed bus being given the low value of U, the depth of field is very high. Practically, it is necessary to use objects small thickness (0,5 nm) in order to be as much as possible transparent to the electrons.

Moreover, the biological samples must be dehydrated if not water present in these samples would vaporize immediately being given the very low pressure reigning in the emptied tube of its air.

Analyse d'une image de microscope électronique à transmission

These photography shows an enlarged yeast cell
20 000 X which divides by budding.

We observe a cut of the core, while in the cytoplasm are present secretary organoids, which make it possible the cell to secrete proteins in the external medium.

Image given by Mrs. Morin-Ganet Doctor of biology which worked on the yeast Saccharomyces cerevisiae in electron microscopy.

The scanning electron microscope carried out by Manfred von Ardenne in 1939 is an apparatus which balie the sample of an electron beam. These electrons strike the sample which emits in its turn of the secondary electrons of which the number depends on nature on studied surface.
These are the electrons which are collected and detected. In the case of the apparatus opposite, the enlargement can vary from 10 X with 50 000 X and its resolution can reach 7 nm.

Scanning electron microscope
ETEC Autoscan (© 1995, ARS)

Optical microscope Electron microscope light beam optical lens resolution : 500 nm electron beam electromagnetic lens resolution : 0,2 nm

1 nm = m