Transmission Electron Microscope:

Instrumental Part:

Lenses (Condenser):

Electron lenses are the magnetic equivalent of the glass lenses used in the light microscope. The behaviour of all the lenses in a TEM can be compared to the action of a convex (converging) glass lens on monochromatic light.

Figure 12.05: Schematic of condenser lens in TEM and its Light Optical Analogue [2].

A strong magnetic field due to the current carrying coils (see green circles in Figure 12.05) acts as a convex lens and bringing off axis rays back to focus.

In a magnetic field, an electron experiences the Lorentz force: F = -e (E + v × B)
E: strength of electric field, B: strength of magnetic field, e/v: charge/velocity of electrons

The focusing effect of a magnetic lens therefore increases with the magnetic field B (by controlling i).
Focal length can be altered by changing the strength of the current.

Second condenser lens affects the convergence of the beam at the specimen and the diameter of the illuminated area of the specimen.

Ref.[2]. http://www.microscopy.ethz.ch/lenslight.htm