Electromagnetic Lenses in (TEM)

Electromagnetic Lenses in (TEM)
Lenses in light microscopes are made of glass; however, glass lenses cannot be used in electron microscopes because glass does not deflect or focus an electron beam. Noting that electrons have electric charges and that electric charges interact with magnetic fields, we can use a magnetic force to deflect or focus an electron beam. All lenses in an electron microscope are electromagnetic. Figure 3. illustrates the basic structure of an electromagnetic lens. The lens consists of a case and two poles. The case is made of soft-magnetic material, and encloses a solenoid. The poles are two pieces located at the narrow annular opening of the case; they are machined to very precise tolerances. Such an arrangement creates a powerful magnetic field concentrated in the gap between the two pole pieces (N and S). Thus, when an electron beam passes through the lens, it is focused and deflected by the field’s magnetic lines of force. Since the strength of a magnetic field can be easily manipulated by altering the electric current that generates magnetic field, an electromagnetic lens has a special advantage over a glass lens: the magnification power of an electromagnetic lens can be altered by simply changing the current passing through the solenoid of the lens.
The lens system of a TEM is more complicated than a light microscope, besides the electromagnetic lenses. There are two or more condenser lenses to demagnify the electron beam emitted from the electron gun. The condenser lens system controls the beam diameter and convergence angles of the beam incident on a specimen. During operation, the illumination area of specimen and illumination intensity can be adjusted by controlling the current in the electromagnetic lenses of condensers. The TEM has three lenses to ensure magnification capability of about 105–106 times.