To further understand how an SEM works, we must begin with the electrons. In a light microscope, light from a source (usually an incandescent light) is focused through lenses onto the sample. The image is formed when the sample reflects and absorbs different wavelengths of this light which is detected by our eyes and formed into an image by our brains. An electron microscope works in a similar fashion. Electrons from a source are focused on the sample. These electrons reflect off the sample (As will be explained later, these electrons don't really reflect off the sample, but for now, let's assume they do), they are then picked up by an electron detector and then processed into an image which is projected onto a CRT that our eyes can see.

     To begin our understanding of how an SEM works, let's begin with the source of electrons, the electron gun.Most SEMs have what is called a hot cathode source, usually a tungsten filament similar to that in an incandescent light bulb. When such a filament is heated by passing current through it, it not only emits light, but an electron cloud forms around the filament. Left on their own, they remain in the cloud and are reabsorbed into the filament when the current is removed.

Place a positively charged plate (an anode) near the filament and the electrons (being negatively charged) will be attracted to it. Problem is, the electrons would not be well directed and would probably jump over to the anode plate in a series of arcs. But place a negatively charged cathode plate near the filament (which they are repelled by) with a hole in it and a positively charged anode (which they are attracted to) under this with another hole in it and we have the makings of an electron gun.

The electron cloud is attracted to the anode plate enough that they will travel through the hole in the cathode. But in doing so, they gain enough speed that most of them travel right through the hole in the anode plate.

Now we have an electron gun. The speed of the electrons emitted from this gun is controlled by the amount of potential (accelerating voltage) applied to the cathode and anode plates.

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