So now we have a beam that is scanning across the sample surface and this beam is synched to the beam of a  CRT. But how is the image formed? To understand this, we need to know what happens when the electron beam interacts with the atoms of the sample.

The incident beam electrons (from the electron gun) do not simply reflect off the sample surface. As the beam travels through the sample it can do three things: First, it can pass through the sample without colliding with any of the sample atoms (matter is mostly space). Second, it can collide with electrons from the sample atoms, creating secondary electrons. Or third, it can collide with the nucleus of the sample atom, creating a backscattered electron.

The incident beam is composed of highly energized electrons. If one of these electrons collides with a sample atom electron, it will knock it out of its shell. This electron is called a secondary electron and is weak in energy (nearly 100 volts). If these secondary electrons are close enough to the sample surface, they can be collected to form an SEM image.

The incident beam electron loses little energy in this collosion. In fact, a single electron from the beam will produce a shower of thousands of secondary electrons until it doesn't have the energy to knock these electrons from their shells

If the incident beam collides with a nucleus of a sample atom, it bounces back out of the sample as a backscattered electron. These electrons have high energies and because a sample with a higher density will create more of them, they are used to form backscattered electron images, which generally can discern the difference in sample densities

To see examples of the difference between
secondary and back scattered electron images click here.

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