What is Electron Microscopy?
The electron microscope is a type of microscope that uses a beam of electrons to create an image of the specimen. It is capable of much higher magnifications and has a greater resolving power than a light microscope, allowing it to see much smaller objects in finer detail. They are large, expensive pieces of equipment, generally standing alone in a small, specially designed room and requiring trained personnel to operate them.
All electron microscopes use electromagnetic and/or electrostatic lenses to control the path of electrons.  Glass lenses, used in light microscopes, have no effect on the electron beam.  The basic design of an electromagnetic lens is a solenoid (a coil of wire around the outside of a tube) through which one can pass a current, thereby inducing an electromagnetic field. The electron beam passes through the centre of such solenoids on its way down the column of the electron microscope towards the sample. Electrons are very sensitive to magnetic fields and can therefore be controlled by changing the current through the lenses.
The faster the electrons travel, the shorter their wavelength.  The resolving power of a microscope is directly related to the wavelength of the irradiation used to form an image.  Reducing wavelength increases resolution.  Therefore, the resolution of the microscope is increased if the accelerating voltage of the electron beam is increased. The accelerating voltage of the beam is quoted in kilovolts (kV). It is now possible to purchase a 1,000kV electron microscope, though this is not commonly found.
Although modern electron microscopes can magnify objects up to about two million times,. Researchers can use it to examine biological materials (such as cells), metals and crystalline structures, and the characteristics of various surfaces.  Nowadays, electron microscopes have many other uses outside research.  They can be used as part of a production line, such as in the fabrication of silicon chips,  they can be used to look for stress lines in engine parts.
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Types of Electron Microscopes

1)Scanning Electron Microscope (SEM)

The SEM is a microscope that uses electrons instead of light to form an image. The scanning electron microscope has many advantages over traditional microscopes. The SEM has a large depth of field, which allows more of a specimen to be in focus at one time. The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels. Because the SEM uses electromagnets rather than lenses, the researcher has much more control in the degree of magnification. All of these advantages, make the scanning electron microscope one of the most useful instruments in research today.
In the SEM, we use much lower accelerating voltages to prevent beam penetration into the sample since what we require is generation of the secondary electrons from the true surface structure of a sample.  Therefore, it is common to use low KV, in the range 1-5kV for biological samples, even though our SEMs are capable of up to 30 kV.
How does a SEM work?
A beam of electrons is produced at the top of the microscope by an electron gun. electron gun fitted with a tungsten filament cathode. Tungsten is normally used in thermionic electron guns because it has the highest melting point and lowest vapour  pressure of all metals, thereby allowing it to be heated for electron emission, and because of its low cost. The electron beam is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface. The size of the interaction volume depends on the electron's landing energy, the atomic number of the specimen and the specimen's density. Once the beam hits the sample, electrons and X-rays are ejected from the sample. Detectors collect these X-rays, backscattered electrons, and secondary electrons and convert them into a signal that is sent to a screen similar to a television screen. This produces the final image.