There are mainly two types of microscopes. One is a Transmission Electron Microscope (TEM), and the other is the Scanning Electron Microscope (SEM). Let us first understand how an electron microscope works?
The TEM was the first ever electron microscope and it was used to see within the cells. The TEM allows only those electrons to be viewed which pass through the subject, which obviously means that only those electrons which pass through the sample may be seen. This gives rise to black/white and shades of gray on the finished picture.
The electron is a pin-shaped cathode, heated to an extent which induces a beam of electrons to be