Surface details which would be impossible to see with a light microscope can be magnified up to 300,000 times. An electric current heats the filament to a temperature at which it emits electrons. A high negative voltage applied to the cathode accelerates electrons towards the anode with some passing through the hole. In a TEM the specimen is an ultra-thin section of material. When electrons hit it most will pass through it.
Some electrons bounce off heavy atoms in the specimen.
The beam of the scanning microscope passes across the surface of a specimen rather than traveling through it. When the beam hits a conductive surface it is split into its constituent electrons at various energy levels. The signal produced by secondary electrons is detected, amplified and displayed on screen in a way similar to that of a normal television image.